I’ve had a dye-sub portable printer, which was utterly terrible (Canon QX10). Printing was extremely slow, the battery exhausted after just a couple of prints (and took ages to recharge), and it broke, twice, before even finishing a single cartridge. A piece of junk and an embarrassing waste of money. Print quality was OK, though, and it printed on sticky-back paper.
I still have a bigger 4x6in dye-sub printer (Canon CP1300), which isn’t exactly portable, but is small enough to transport, if that makes sense. Print quality is as OK as the above piece of junk, it’s as slow, and the app (the only option for printing) no longer works except for one particular phone that still runs an old version. Still, this has been with us for many years, has proven reliable, and has served us well. I once brought this to an event I was photographing, and the hosts were delighted.
I also own an instax square printer, which works well. It’s compact, portable, fast. Print quality is mediocre, but has some analog charm. The only downside really is that you can’t cut the prints. I once brought this on a group camping trip, which produced some cherished artifacts for everybody.
A colleague recently showed me their ZINK printer, which seems to combine the better print quality of the dye-subs with the robustness and speed of the instax. If I were to buy a mobile printer today, that’s what I’d buy.
Although truth be told, even my very modest six-ink office inkjet printer (Epson XP8500) utterly obliterates any of these in terms of print quality, versatility (different paper stocks and sizes!), cost, and speed.
My photo printer (Canon Pro200) is a different beast altogether. But now we’re also very much beyond reason for most people in terms of size and price.