A fast difficulty adjustment works both ways. Say a lot of SHA256 power goes into bch...it rapes mining for a few tens of blocks with low diff and then miners switch back to Bitcoin, while bch hangs there, waiting for miners, but nobody is mining it due to high difficulty. It'll take hours to get it unstuck.

The effect on bitcoin is a few delayed blocks, while the fast-adjusting bch will have created an anomaly similar to how multi-pools rape altcoins - while getting raped itself and then left for hours to come down. The altcoin market has shown that the fast-adjusting coin is more exploitable and disrupted from multi-pools.

The problem cannot be easily solved for bch because if you try a detection mechanism that checks whether the blocks aren't arriving to do a massive diff cut, then you are going into an uneven mining pattern, accelerating inflation.