I've been involved in a graphql project in the past and it is superior to a RESTful api in my opinion. More flexibility.
So in terms of REST vs graphql, graphql does everything REST can.
The main issue is that saleor graphql is not ready.
I also would be happy to be able to use the graphql api today as I'm starting a new project soon. I suppose I just gave up on the using the graphql api for now. The roadmaps shows the items to be completed and I think it's a substantial body of work. I suppose if you can afford to wait for a few months that might be your best bet.
GraphQL def seems like a good technology but it requires some up-skilling on the devs' side and it being in beta is not a good story. I just thought there'd be a REST API in place catering for all the other guys/girls that wanted to build a react/vue/angular frontend
I'm not part of saleor, in fact, TBH I'm still evaluating it now for use in an upcoming project. I'm also not a python developer which makes life a bit harder for me. I'm here for the same reasons you are :-)
For the mobile apps, you will need the graphql api alright. Come to think of it, I wonder if there is a demo graphiql page where people can come in and test out the graphql Beta. Question for the core team.