Just seems like not a very smart long-term move to build on it.
Tough to get changes upstream when the majority of engineers working on Chromium are Google devs.
Lots of features you'd hope would be available in Chromium aren't there and have to be implemented manually, but then you need to keep your fork interoperating with a massive, moving target. Safe Browsing, Translate, Spellcheck, Autofill, Password Manager aren't available in Chromium and Google cut unauthorized Chromium browsers off from using Google Sync in 2021 (https://www.zdnet.com/article/google-cuts-off-other-chromium...)
There's probably more issues?