I am having the same issues with the libretro mames.
Some games select works as the default coin button and some do not. Some will only allow coins to be inserted on the player 2 gamepad. Even when I manually re-map the coin button from the in-game menu it doesn’t work.
Also, I’m not clear on how to configure retroarch per cite as the tutirial floob made is outdated and the retroarch menu options are different.
Mame is, by far, the most frustrating aspect to configure on RetroPie.