Carracks are undeniably as situational as resourceless knights, if not more so. Good luck gaining much utility on pangaea or even continents.
Vanilla cossacks were godly unstoppable death units. BTS cossacks? How exactly is that an awesome UU if the camel archer isn't? Cossacks get NOTHING against their counter unit, the rifle. That's what they'll be fighting most of the time ----> if they can come up against cavalry, they can come up against rifles. Vs anything else they're essentially cavalry. Granted, cavalry is great, but what makes the cossack so much better? At least the camel archer gets a bonus against what it will actually be attacking relative to the stock unit.
And I can see why nobody wants to touch the bowman/ballista elephant argument

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Can't say for certain as I've never tried but wouldn't a bowman be very good at choking other civs? Whenever I've tried with other non sitting bull archers they've always been killed by axes, so the 50% against melee might make a considerable difference.
Ballista ellies I don't like at all, for me they tend to backfire when I use them. I want to kill the maces with my shock ellies not the damn chariots!

I have had occasional use out of them, but only on defense where the BEs can actually catch the enemy stack, they are imo the worst UU.
Barring the early anti-archer type units, what else qualifies as incredible to behold? Cossacks in bts? Redcoats vs anything but other rifles/muskets (aka a tech lead war)? Samurai? Gallic warriors? Panzers? Holkans against competent players or high level AIs? The bowman? I can go on if you like. Knights are the strongest base str unit in their era, so giving them survivability is a boon. Of course cataphracts are better, but that doesn't make CA's a bad unit. Flanking CAs can be quite nice as part of a stack, and if you're going the mass horse archer route then the withdraw guys are more palatable as survival strikers than stock knights.
It's not going to win awards for best UU ever, but the withdraw is a useful bonus and puts it ahead of frequently horrible UUs like the ballista elephant, bowman, and panzer.
Yeah I agree on panzers, IF you ever get them the tech gaps have usually opened up to such a degree that they will rarely end up fighting against tanks and against everything else they're just normal tanks.
Samurai I'm not a fan of but they're ok.
Carrack on pangea can be nice, they don't get ejected from borders on DOW do they? Also you can still build them until at least past combustion (Do caravels EVER obsolete!?). I can see them making for some interesting sneak attacks in the right circumstances. Although I can see this is a niche use.
I quite liked Gallic Swordsmen in a game I played recently, Guerilla 3 is seriously underrated imo and as far as I know Gallics are the only melee unit able to get it. +50% defence on hills protects them against axes, 2 moves over hills makes them very good at worker capture farming, and 25% attack against hills nullifying the bonus (well would be better if the AI didn't use solely archers as garrison but stilll nice, and obviously this all relies on hills but they can't be chopped)
Most importantly a HUGE 50% withdrawal bonus. I think this is enough to allow a fair number of useful strategies to be used using Gallics, Guerilla 3 maces anyone!? I just wish the Dun lasted longer

it could be killer post rifling.
This is exactly the kind of thing I like in my uniques, the ability to use them in ways that would be impractical even impossible to the standard unit. I kinda find the CA boring, wouldn't say its the worst UU though not by a long shot