Babri:
AI loves to get drill promotions & fortify units in forests/hills. If you don't have rough terrain specialists then you are in trouble as it will slow you down. And if you go for rough terrain promos then you loose the quicker high tier bonus advantage for those units.
Now janissaries not only have attack bonus but I also give some of them drill promos so they can easily overcome those units fortified in hills/forests. And it works perfectly, I alternate turns for the killing so most units have almost full health.
The AI actually loves to fortify pretty much anywhere you attack it with ranged units. Once it does so, it's essentially a sitting duck. Your entire army won't be made of up of melee units. A substantial portion will be ranged and siege. Alternating turns for kills only really works if you already support your melee units with ranged, in which case I find the damage the melee does to be largely supportive rather than defining, even for Janissaries. As mentioned, some of these units will have drill and will have the terrain bonus. A Janissary, even with Drill, should mostly be at par with such a unit. There's no way you're killing it with two melee attacks.
So either most of the damage is being done with ranged, in which case having Drill Janissaries is a weak advantage, or you're taking a lot of return damage that isn't being healed, in which case Samurai with Medic2 have an edge. It's just one or the other, neither of which makes Janissaries a strong advantage.
In my experience AI fortifies unless it has superior forces than yours. And you will be required to fight some battles in hills & forests. But this maybe subjective.
If you cause it to DoW you, it will usually be because it perceives that it has superior forces. It will absolutely attack you in your territory then. You can choose where that battle will be. If you have extra GG's via Samurai, you can make it absolutely one-sided, even at a tech disadvantage.
My point was that janissaries are far superior than samurais at field battles, samurais on the other hand get an advantage in sieges due to quicker access to siege & medic promotions. Janissaries however can get these promotions just before going to siege by killing a couple of units. Yes samurais can do the same but that is not my point. Not only janissaries own most of the units of their era due to 25% bonus attack but they also heal while killing, making enemy perfect for farming some XP so you get those promotions you need before you go for the difficult sieges.
I totally get that. Yes, Janissaries can do that. They're powerful in that way.
So far, all you've managed to say is that Janissaries have a slight advantage vs. Samurai vs. attacking fortified AI melee units in Rough terrain, none in open terrain, and are disadvantageous in sieges. I don't see this as "far superior in field battle."
Siege, Medic, Cover 2, and multiple terrain bonus units all are good in both field battles and sieges. Do not forget Cover 2. It is absolutely game-changing with Medic2. A Samurai Musketman can conceivably tank damage from an Oligarchy-boosted 40+ defense city using Cover 2 and supported by Medic 2, especially in friendly territory (courtesy of GG claim, natch).
I do not find +25% attack to be fantastically good - less powerful than Drill 2 or Shock 2 numerically, and doesn't activate on retaliation attacks or ranged attacks. Even if you attack a Janissary on a unit the AI fortifies on a hill, unless you were about to overrun that position anyway, that Janissary will face retaliation attacks the turn after from the city, enemy ranged units, and the like. Neither Drill nor the Janissary's bonus will factor into that. Cover will.
Samurai get bonuses that allow them to be very effective immediately. More promotions add to their efficacy; it does not detract. If I may be so bold, perhaps the perception that the extra promos are useless is because there isn't as much emphasis on functional multi-promo units and multi-promo interactions on the field? Many posters, not just yourself, continually miss the significance of these.
You imply here:
And Samurais don't get march out of the box so you'll need to work on it just like janissaries.
That a bunch of level 4 Janissaries and Samurai would have largely leveled the playing field. That is not at all the case. At the point where you're getting Siege Janissaries, I'm getting March Samurai (and I still have basic Siege Samurai). March Samurai with Medic 2 support are pretty much fantastic for anything, since they heal 20 HP every turn, on any location. Drill or Shock is almost incidental to that sort of advantage.
Once again:
At the end of the day what will you prefer, some level 6-7 janissary guys or samurais?
The implication here is that the extra promotion advantage for higher promo units or a generally higher promo army is worthless. Not so. Not only can I afford higher promo units out the gate, I can also spread the promos so that I have more higher promo units in my army.
Level 6 is 5 promos. A Samurai will have 6. So, March and Siege? I'm not sure how you're arranging your promotions as you haven't detailed the strategy you use for coordinating your promotions as you advance.
Offhand, I would like (and have made) Samurai starting from Cover 2, then go to Siege, then Amphibious so I can just ignore rivers, especially when taking cities. This is an absolutely potent unit. Amphibious is not that good on its own. Combined with defensive and offensive promos on a siege unit, it's a remarkable advantage, as it can not only ignore rivers but also embark and take cities from the sea effectively (Cover 2 even defends it en route). The unit can also arrive at this point from Siege units - Siege to Cover 2 to Amphibious.
Alternately, March+Blitz+Siege. Once again, a monster. Every end-promo in that equation is necessary to make it so.
If you like, how about March+Blitz+Woodsman or March+Siege+Woodsman. This promo line is actually much more useful with Minutemen, but the added maneuverability on the right terrain works quite well, even without Drill. I'll point out here that Janissaries don't get as much out of March as it overlaps with their UU power.
So yeah, I would really much rather have the level 6 Samurai here. Level 7 involves a lot of XP. Level 6 to level 7 is 210+? For every level 7 you have, I could have an additional level 5 and level 4 at once. At those promo levels, the Janissary's advantages become somewhat incidental. The defining advantages of a March+Blitz+Siege Janissary won't be that it heals when it kills a unit, nor its +25 on attack.
My main thrust here is that as you gain higher promo units, the base advantage of the UU dwindles in significance. The Janissary's key advantage here is that its UU abilities allow it to get there faster. So does the Samurai's, but in a different way - it reduces the cost by 1 promo, which is very material for levels above 5.
I think even if you disagree that samurais are average UU, you have to admit that they are bland & have boring bonuses. Samurai IRL was a powerful & flavourful unit, there is a lot of room for improvement. For example they could get Bushido as a promotion while Japan's UA gets scrapped & changed completely.
I think "bland and boring" comes from the mistaken impression that the Samurai's bonus promo is Shock 1. That's what it says on the tin, but that's not really what the ability does in practice.
In practice, the Samurai's extra promotion is Cover 2, Medic 2, Siege, Jack combos, and the ability to get higher promos faster. Let's do some math here.
15+30+60+100+150 for a level 6 unit. 30 XP is free. You'll see here that, out of the 'Rax, a normal unit (such as the Janissary) requires 210 XP to get to 5 promos at level 6. A Samurai only needs 160 to get to 5 promos at level 5 - nearly half. It's like having Military Tradition automatically for melee units, only your benefit isn't 50% more XP - it's nearly +100%!
The flavor this evokes is of a highly organized and professional military. This is neither bland nor boring. In fact, it's one of the most interesting promos, partly because the advantage is so nonobvious, and it's so interesting to manipulate on both a unit and an army level. In truth, I don't think this is appropriate for Samurai. It feels more Legion-y.
EDIT: I'll say this. If you're promoting all your Samurai Shock 3, and half as Jack into Drill2 Shock1, you're absolutely not getting what you paid for.