Basically hearths are getting species-specific upgrades which also makes organizing who moves close to which hearth easier (so slightly more micro early game, but less late game in each stage).
Also some assorted stuff like new cornerstones and so on.
Still pretty early in the game - on the fourth seal - but I love it. Strong “just one more level” energy, plus the constant cycling fixes the city builder problem for me, how it all got stale very quickly. Here it’s all done after 20-60 minutes each time.
I feel they smoothed it all by now. AFAIK the only bigger issue I recall is that there is sometimes an issue with woodworkers delivery because of how some woods can proc a ton of different extra items, which doesn’t gel well with how delivery pickup is done. Fixable by always using a harpy as a hearth keeper, which I like the most anyways.