Hm, I think I tried it once and it just crashed immediately on a simple binary. Is this r2's debugger or gdb/lldb?

It's r2 debugger that supports native mode for all platforms, remote gdb/lldb, remote WinDbg, and a few more exotic options. Both Cutter and radare2 are developed with quite a pace, if you meet any bug - reach us and we'll try to fix it as soon as possible. This year we were accepted into Google Summer of Code 2020, and students will work on a few interesting projects [1].

[1] https://www.radare.org/gsoc/2020/ideas.html