There are so many things wrong with LaTeX2e according to the latest discussion, Why is LaTeX3 taking so long to come out?, from documentation to speed, inflexible kernel, etc. We are perhaps waiting in vain for LaTeX3 and expl3 full deployment but what is called "gentle re-fractory" see What's the current state of LaTeX3 (2020)?.
What are the deficiency and features in LaTeX2e that LaTeX3 project hence "gentle refractory" and L3 programming seeks to provide solution to (and how significant )? Surely separating design and codes will be great, but LaTeX3 project must prove it's relevance in terms of quality of extension, alternate solutions, user interface and better features than LaTeX2e!
Secondly, I haven't seen any basic and compilable Tex for mobiles OS like android. Many use online Tex Editors and tools or snippets like Overleaf. Will need latest implementation of TeX on say android OS and MacOS.
In my opinion, LaTeX3 project has taken so much time, (more than 20years since the project started), and surlely new issues of change due to time will continually postpone "getting perfected for release" Sure, it won't be able to replace LaTeX2e and conTeXt but "soft refractory" for possible modernization I find information on the following discussions partly useful and educating..
Am currently working on mobile implementation of TeX-Latex.