I built and sold a company built largely around FFmpeg and this type of tool would've been really useful for understanding how FFmpeg's filter chain works. If you're interested in some non-trivial OSS examples, check out https://github.com/transitive-bullshit/ffmpeg-gl-transition and https://github.com/transitive-bullshit/ffmpeg-concat.
One non-obvious piece of advice I have for developers delving into ffmpeg's filter_complex is to try and keep the filter graphs as short and simple as possible. Creating complex filter graphs programmatically (as ffcms does in this case) is super powerful, but I've also found in practice that larger filter graphs often lead to random, impossible to debug errors. This could be due to anything from some combination of VM resource constraints, internal ffmpeg leaks that compound at scale, etc etc.
My advice is to think carefully about what you want to use ffmpeg for. It's excellent at transcoding and doing minor filter graphs but trying to get too crazy with complex filter graphs will lead you down a dark path that imho ffmpeg wasn't really meant for. If you can break up your complex filter effects into smaller, isolated ffmpeg commands it may be slightly slower but your rendering pipeline will be significantly more robust.