For Debian it was extremely painful. A few people probably burned out. Lots of people pointed to source-based distributions and said "they will have it very easy".
Do you have any references that elaborate on that? From an outsider perspective, the time64 transition in Debian seemed to have been relatively uncontroversial and smooth. Way better than e.g. the /usr-merge.
https://wiki.debian.org/ReleaseGoals/64bit-time
I'm continually impressed by Debian. My Debian systems are pretty boring, so maybe it's to be expected, but as an end-user neither the /usr merge or time64 transition broke anything for me, and I'm using Testing.
I did the transition for m68k, powerpc and sh4 and partially for hppa with the help of some other Debian Developers here and there and I'm still alive ;-).
'easy' in a way "just tell the user to rebuild everything in one go"? :)