I was lucky enough to work at a company with a great code review tool at one of my first positions -- and I am nowhere near convinced that GH's review interface is the simplest possible.
Of course, everyone (including myself!) is probably biased to think that whatever they are used to is simplest, tbh.
I agree, you want a basic tool for most users that just need the ability to review the code. But, once your team is doing something where the code quality is important, you'll want to switch to a better tool for the job (like Reviewable!), just like when you switch from a whisk and bowl to a kitchenaid standing mixer when you start baking often
