Apparently they are hard to hack, yes. The SFC Goemon 2 has been in development hell for years now.
Yuki-hime's Disappearance has uncompressed text in all versions, meaning a re-translation is possible.
Shogun McGuiness/Shishi Jirokubei/Kirakira Dochuu have compressed graphics and scattered text pointers, but nothing too wild.
Hoshizora Dynamite (GBC) has compressed text (probably LZ if it's anything like the Hunter x Hunter/Survival Kids GBC games) and some graphics.
Ganbare Goemon 5: Uchu Kaizoku Akokingu (PS1) has uncompressed text and fonts and somewhat weird but not too hard to figure out text pointers, the big problem is the 12x2 font, the 2-byte encoding, and the graphical format.
The main hurdle blocking any translation effort for the SFC games would be the graphical compression (to get the font in).
Said graphical compression was figured out for all four games by various people (with even proof of concept videos and screenshots produced) but they abandoned their projects and didn't release their findings and there was quite a bit of wheel reinventing (for 2 and 3 in particular).
They're not entry level games, but not exactly hard/impossible to mod games to the extent of Sakura Taisen/Tokimeki Memorial fan-translator-nightmare-tier stuff either.
Besides that, there's a lack of interest.
And some have that strange idea that the story is impossible to translate.