On the minus side (and it's a big minus), it's not open source. As a developer of commercial software I basically have to trust the Fork developers not to do anything fishy with all the source code / IP that I am opening up to the app.
Fork is made by one person.
Not making any assumptions here, but if you're evaluating risks around compromises in one of the two products, I think Fork is higher risk.
The business model of Fork is unknown, so to me it's a clear no-no. Can't afford to put a tool in my workflow not knowing if in a few months it will become yet another service I must subscribe to
Android/LineageOS is Linux.
Just type adduser and sudo, it's not too bad.
You'll probably find, however, that it might not be all you were looking for. It can still steal your source code and credentials, which might have been what you were worried about.
You will also miss out on your editor, compiler, zip utilities, and whatever else that's not included in your git client. Anything useful would probably look more like privileges (where opening a file can be ok but a network socket forbidden). There's been a ton of research in this area and some very mature systems such as SELinux.