Quite a lot of testing in Final Cut and Logic etc showing very good performance and efficient swap. So for most consumer apps it works well. No magic if you need a big block of memory for ML or analytics.

Do we know what benefit they're getting by putting the memory on the chip to begin with? I had initially assumed there was some non-trivial performance advantage in it, but now people are saying not so much, so why did they do it?

It seems weird to artificially limit the max memory if there was no advantage in it.