IMHO, ROOT[3-5] is too many things with a lot of poorly designed API and most importantly a lack of separation between ROOT-the-library and ROOT-the-program (lots of globals and assumptions that ROOT-the-program is how people should use it). ROOT 6 started to correct some of these things, but it takes time (and IMHO, they are buying too much into llvm and clang, increasing even more the build times and worsening the hackability of ROOT as a project)

Also, for the longest time, the I/O format wasn't very well documented, with only 1 implementation.

Now, thanks to groot [1], uproot (that was developed building on the work from groot) and others (freehep, openscientist, ...), it's to read/write ROOT data w/o bringing the whole TWorld. Interoperability. For data, I'd say it's very much paramount in my book to have some hope to be able to read back that unique data in 20, 30, ... years down the line.

[1] https://go-hep.org/x/hep/groot (I am the main dev behind go-hep)