It's been a fight the whole dang time. The lead dev is a shite programmer with a gigantic ego and a control freak. He was infamously awful on the emulation subreddit. Deeply disrespectful to the actual emulator devs. MAME team especially have a long lived hatred of Retroarch.

Occasionally he'll rewrite code that was working fine and break something that previously worked. Like...NETCODE. For example. Broke it and refused to take the L. So the guy who wrote the netcode backend quit out of frustration. That's just one of a billion examples.

His ego is rivaled by the likes of stenzek (Duckstation emulator, and front end wizard for Dolphin and PCSX2) the guy behind Simple64, and TeknoParrot. Except those guys are actually good programmers, just insufferable.