I’m working through the entirety of brilliant.org right now; some of its courses are specifically maths, and the skill level goes from introductory courses all the way up to maths-degree level topics.
My take: it's good but has some drawbacks. The biggest one for me is that the questions are mostly multiple choice. You don't have to show your work or reason through the problem, so it's easy to mentally cheat and say "eh I think it's this one." This isn't so much a fault of Brilliant as a limitation of the smartphone/web technologies vs. pen and paper. I've been having some success with a blend of Brilliant, books, and application (using what I learn to read some physics). I don't think using it on its own will probably be enough though if you want to be able to apply or know the topics deeply.