For consumers of the information and those paying for it, it’s all upside to me. My only prediction for downside will be increased author fees for open access publications. Some venues have ridiculously high fees for open access authors, which is a barrier for some (not every author is funded by a research grant or in a department with a budget that can cover such fees). I expect they’ll go even higher, and the available exceptions or discounts will be more stringent. To me, the upsides vastly outweigh that downside though, so I’m very happy to see this move.

Some society-level journals that are used to help support their respective societies are likely going to struggle a bit.

That doesn't mean this isn't worth doing (it is), but it's going to be a thing.

I don't see any to be honest. Public money, public access.

the publication fees (paid by the authors) seem to be considerably higher

Definitely agree it's a good thing - my libertarian core can't help but wonder if the white house has (or ought to have) the power to unilaterally declare this, though. Would much prefer this had been voted on by congress.