For HU poker specifically: from what I recall, researchers were having rough upper bounds on how far their model is off from Nash equilibrium. It was down to something like less than 1 big blind(BB) per 100 hands or so, implying someone playing perfect NE could exploit such a bot by no more than that amount. So if you're playing 100$ BB HU, you'd hope to make at most 100$/100 hands - so not exactly a get rich quick scheme. I'm most likely off about the exact upper limit here, but I recall it being so small that for all practical purposes, HU poker is considered solved (the NE, that is).

Chess is different as the game tree is way bigger, you can't just lump possible moves together, some tactical lines only reward the player 7+ moves in the future, etc. No limit holdem has a lot of leaf nodes (all-in & folds) and a tree depth of about 7 on average or so. It's crazy how much more complex chess is. Think of how to this day we don't even know what's 'the best' opening; there are a few hundred perfectly playable openings (as ranked by AI), leading you down on a completely different gamepath each time.