One downside of PICO-8's closed source is that it can't easily be ported to different architectures by fans, so you're limited to Windows, Linux (x86) and the Raspberry Pi (and HTML/JS, but just the player). There's no Android port.

TIC-80, on the other hand, runs more places (Android!). And someone has ported it to run on OpenDingux portable handhelds, there's a RetroArch core, etc.

That said, PICO-8 is super, super polished and absolutely worth the $15.