Kernel dev here (my own project):

ARM is great. Love it. There are tons of chips that all do things differently, much more so than x86. Plus the booting mechanisms are a PITA to figure out, debugging is tricky, etc - and that's a single target. x86 isn't particularly elegant given its years of cruft and questionable design decisions but at least it's quasi-consistent (yes, lots of "well actually" possible; I'm speaking broadly).

The ISA on ARM is better, IMO, than x86 but the surrounding ecosystem is hellishly fragmented. Just my $0.02. makes it difficult for OSdevers to find an 'in' to a wider ARM audience. rpi is a great entry point.