I don't recall having problems with ACKs but I might not have gotten as far as you. I realized after a while that the 6502 was doing too much work managing the display and it would be simpler just having the smart display manage it (backspace comes to mind). And if I recall there are some things you just can't do in the Serial Environment (or was that just the Serial Commander App?). It was likely at that point that I changed my approach.

Once the ACK words get out of sync, you crash.

$15 is NACK - the error code you need to worry about.

Ah, I never got that far. Sometimes I wish I could simply call those serial functions from the 6502, but that's not possible in the Designer environment.

According to SparkFun, the Diable16 is discontinued, but the Picaso isn't? It seems, perhaps, that Picaso is more popular? Diablo16 is clearly the superior chip.

Maybe just from their catalog? Plenty of Diablo16 products at 4D Systems. Maybe they're keeping all of the chips for their own products. There are a lot fewer Picaso products there, so maybe more chips available for sale to third parties?