The only question that really needs to be answered about Dumfries is: "can he defend well?"
We have all seen how effective he is going forward. Great. But his primary role - by definition - is to defend. This is where he will be judged until Everton have enough technically superior players in the middle of the field to be primarily a proactive, ball-hogging attacking side.
My fear is that Evertonians are conditioned to look at their full-backs only through the prism of attack. Seamus Coleman was never a great defender - but he was, in his early days, eye-catching going forward. Leighton Baines is renowned for his overlapping attacking play, his free kicks, his cool penalty kicking ability, and his assists. He was a very ordinary defender. Similarly, Lucas Digne. Nobody raves about his tackling, positional play, or defensive excellence. It's all about what he does going forward.
This is all very laudable, but it really only makes sense in the context of an attacking, dominant side who has the ball so much that defending is really a function of possession. Until we transmogrify into that kind of side, there will always be questions about the effectiveness of attacking full backs when we need people who can actually defend.