Is there anyway to know what features ardour has when people download the nightlies
Short version is not really.
Longer version is you could read the changelog for git, and go through the messages to know what should or should not be there, but just because a commit message exists mentioning the feature doesn’t necessarily mean the feature should be there.
I think it has been said in other places already… but the main thing the nightlies have is the ability to break a session as they are based on git master. So the nightlies will not load a session made with Ardour 5.12. Even a session made with the nightlie one day may not load the next. They are pre-alpha.