the MegaCD, FAIL all the way through
It's hard to understate just how mind numbingly bad the MegaCD was, little more than a bare CD drive with no useful firmware or tools, most annoyingly no file system - you had to write your own ISO driver and file system builder. Acceleration functions barely faster than using the 68K CPU, probably unavoidable in a system without direct frame buffer access.
A system doomed for failure, painful to work with and I was very glad when the project I'd been working on got cancelled.
The FX chip had a big advantage, even sitting on the wrong side of a cartridge, it had more direct access to memory, instead of injecting graphics through a narrow write port. Hardly surprising it did the job so much better. If they really did pull out of Playstation because MegaCD failed, that's really unfortunate, it should have performed much better.
Still, we did get the Sony version out of that withdrawal.