I also have to say nerfing mines is a really! bad idea imo.
That's the wrong way (as in using a big berta versus a gnat.).
And I'm rather sure of it.
The existing good improvements (after! the nerf to cottages) weren't to strong (by a long shot imo, i have not even prioritized mines up until now even though they were quite useful. And i did just fine in my games. From now on i'll go out of my way to avoid them and hilly terrain.), the advanced ones were to weak (and by a long shot).
Balancing by reduction is only really useful if something is broken.
Possible real fix for the underlying problem: Put mines back up to +2 hammers and put a third progression of the advanced improvements (so an additional +1 hammer on Lumbermills at Iron working, an additional +1 hammer for Workshops at Machinery, and an additional step for improving watermills.)
Yes i'm seriously thinking that ultra-late-game very powerful improvements are fine. And im rather certain of it. + I think that the potential damage of that is miles less problematic than what is proposed for the upcoming patch.
As is, nerfing is the wrong (plain and simple without much uncertainty) option. As in making the game less fun.
Karnja and Monkeyfinger have outlined some of the reasons.
But that's a good illustration why calling something overpowered as soon as something seems to work nice is actually a bad idea. Be sure you don't get more than you bargain for with the team. Before! actually making a call.
Other problems:
Dwarves just got seriously nerfed and Arete just got seriously nerfed.
Possible hotfix: Dwarven Palace either gives + 1 hammer to Hills or to Mines.
Give Arete +2 Hammers for Mines. (only problem whould be a combiniation of the 2. Not sure if that's worse than what is proposed in the changelog though.)
No dwarves are not much stronger than the elves. On the contrary. Anyone seriously doubting that one? Now on that note, elves not exactely got stronger with that patch. But will suffer a lot less than others. Especially dwarves.
FoL + serious Micromanagement just got improved further (not exactly the worst religion up until now. Especially glaring in comparison to RoK/Arete which was even a bit weaker before usually. For most civs.)
Also the Flavor is now gimped because the high-tech-improvements are actually better from the start. With FFH2 being medival fantasy mostly. Not steampunk. (At least until very late into the game.)
All in all a nice recipe for community disorder if you ask me. (and this time it whould be justified. As in the game will be objectively worse after the change.)
@ Kael + possibly Team?:
Perhaps instead jumping on any notion from the community lately
(sorry but that change very much looks like it, because i can't spot all to many voices seriously calling for that change. At least not from someone who did seriously consider the consequences),
you should start actually asking the broader community what they think of such
(like you did with Chariots and the magic system) prior to undertaking such a massive change.
(i assure you both are actually marginal / minor! changes in comparison.)
This one might be the most! serious change of a core aspect of the mod in the history! of FFH 2. Ever (and there have been zounds of modifications so that is telling something...).
Which negatively effects every! single! civ in every! single! game which is not lost in the first 50-100 Turns and even 2 Religions in addition to it + city placement (and early religions which are seen in most games to add to that.).
And in that case here it wasn't even as strong a cry for nerf as usual... Nearly uncalled for and adding completely unnecessary complexity for its own sake.
I can assure you here that the majority of the community would rather see the other improvements changed / enhanced later in the game than to gimp / change the balance of the improvements / flow early game.
(anyone care to do a poll to show what i will predict will be a 90/10 ratio against such a change?
Maybe I'm totally mistaken but i have serious! doubts about that one.
But! if you do:
Please! take care to do a good! one with many balanced! and well! thought out options! not just black and white polarization (as much as i personally feel polarized on the matter). The thing at hand is a very! serious matter for the mod so any time taken to do a stellar poll with much contemplated options is not wasted. I assure you.)
Honestly farms have always been among the top contenders of any improvements (especially after cottages got nerfed) in question with the many uses for food in FFH2 are you seriously considering nerfing those as well?
I see a negative nerf-spiral of the worst sort coming our / your way (and i feared something like that might happen when cottages first got nerfed with which I overall can understand. Looks like fear just materialized.) in the coming discussions if the change goes ahead.
I don't think this is remotely a good idea from any angle. The mod has precisely been fun because! good improvements are available + useful for the whole game and not just in the end. The call here was to make late game improvements more viable (And i think it was basically exclusively about late game. Maybe im starkly mistaken? Early game = mostly fine as is.
It isn't broke by any means? Don't fix it.)
Reverse that and you have: balance ---> fun (both very much in bold). Are you sure you'll want that for your mod?
Don't get me wrong. Its your mod, its your call.
I like it much.
I'll likely continue to play it (especially because i'm a player not all so much overfond of mines. I rather dig farms + cottages and civics which use food and other things for production / recruiting units / generating hammers).
For the rest of the community on the other hand (especially new players and players enjoying shorter games in terms of time and turns), I'm far from so sure.
But i do precisely speak so clearly and starkly because i care and I'm thinking you'll do a potentially devastating thing here ultimately eating alot of your time + resources for trying to fix a relatively minor problem (big bertas and gnats come up again.

) with a major long-drawn balancing mess where a good outcome is far from certain in the end (most likely far worse in the beginning) when so many other far more pressing things await (AI-issues come to mind.).
(yes, improvements are an important aspect of the game. Yes its worthwhile to tune their balance so most are viable. Very much so.
But when the cure for the "minor cold" of underpowered late game improvements is the "black death and pocks" of junked early game + broken flavor and i'll rather stick with a "minor cold". Thank you!
And I'm not even digging into short games so much like many other more aggressively minded players...)
Edit: tuned down a bit so sensibilities are not touched that hard...

On review, I guess its better to stick to "dead" plagues instead of modern still raging ones.
If your real issue with mines is that they get way to powerful with gunpowder (i don't exactly agree here, but that notion seems much! more reasonable.) then by all means axe +1 hammer for mines on gunpowder so they don't overshadow other improvements anymore. Lategame!
Not the best of changes imo but miles and away better than what was proposed for next patch + that's much more up to opinion and can be a very reasonable debate where i can very well understand all sides of the argument (with me being in the camp of people who say late-game improvements need a boost not the rest a nerf / less improvements over the cause of the game.).