Brilliant doesn't have spaced repetition, it has much less coverage and doesn't have enough exercises to let you really master subject, but it's an easy to use mobile app with nice visualizations that can give a conceptual understanding of many things. If MathAcademy and Brilliant covers the same topic, Brilliant may be more visually appealing and contain animations. Brilliant is nice for getting overview of many things, but doesn't really go that deep.

MathAcademy is great for learning new topics that you haven't studied before, provided they have a course in that subject.

I've tried both and am currently using MathAcademy to learn new knowledge. It's very, very good at incrementally building out concepts and gradually adding complexity. I had no luck at all with Brilliant. MathAcademy practically forces you to bust out a pencil and paper and dive into problems, but Brilliant was more like watching a kind of neat YouTube video on a topic.

I reckon it can and I am about to give it a shot, myself. It sounds like a Duolingo sort of structure, which can be boiled down to "use it or lose it" which is not hard to support with compelling evidence. Say what one will about Duolingo, but my Spanish and German are passable because of it (no, I am not fluent, but I can get by in common situations). Ideally, MathAcademy will cultivate the same results.