On one hand it's great that it was written in Python and fairly extensible thanks to that, but on the other it's a shame, since it probably will never be fast enough for high performance symbolic computing.
That said I think CASes (Computer Algebra Systems) are heavily under-utilised nowadays. Schools and universities should use is as widely as possible i.e. it should be even more widely used than calculator is/once was. I believe that for every two hours of regular pen-and-paper exercises there should be at least one hour of CAS assisted computer exercises. It's pretty radical and sadly there aren't enough teachers (even in good universities/colleges), that know CASes enough to teach that, so it isn't gonna happen any time soon. Maybe I'm biassed here (I hated doing repeated exercises, such as algebraic simplifications, fraction simplifications etc. in high school), because I use CASes as crutches for doing tough math, but I believe sciences would benefit if more papers in math and physics was written as a code (less mistakes, plus it's not a mistake – it's a bug!). Hopefully it will happen someday and we will have "open source derivations" of formulas, theories, theorems etc. :)
That said, the thing about CASes I'm missing most is more hand guided, step-by-step and mathematical transformations assistant approach. Recently they added a bit of this kind of functionality to Mathematica 12 (sadly I use it most of the time, despite it being closed source) – functions like AddSides, MutiplySides, ApplySides – but I think it's still too automatic for what I have in mind. Hopefully when I finish my PhD I will have some time to work on the kind of "assistant CAS" I have in mind using projects like SymPy, Maxima and Reduce for some parts. What I have in mind is to make hand-like derivations using CAS and let it do small things, like I want to substitute log(a) as x in integral, so find dx for this substitution (actually SymPy has this functionality, even though slightly limited at the moment! [https://github.com/sympy/sympy/blob/58e1e9abade7c38c66871fd0...) and do more things this way (series, equations, inequalities, derivatives, limits, operators etc).
Yeah, it's probably more suited for series of blog posts than HN comment but if someone knows FOSS software that is focused on this kind of approach I'll be thankful for links to articles code etc.