It's a very very serious downside for me that there are now two MySQL's (three if you count Amazon's Aurora), just like there are multiple Linuxes and two Pythons.
Isn't the number of pythons four? One for C, one for JVM, one for CLR and one in pure Rpython for the pure JIT code. Double them for python 2 and three. That makes it 8.
There are two (very closely related) languages. In principle implementations are compatible, whereas language version are explicitly incompatible by intention. I think that was clear from the parent.