Most are system-on-a-chip implementations with only okay compatibility. Color palettes will be slightly off or sounds will be a slightly wrong pitch, won't support all carts, etc.

Your best bet for playing your games on a modern screen is to get an FPGA based system, a top loader NES modded with HDMI output or simply use a cycle-accurate software emulator on a computer.

I'm interested in them mostly because my NES, SNES, Genesis and OG GameBoy all still work, and it would be nice to be able to still use my cartridges once they finally bite the dust. I got real lucky with the NES getting refurbished by Nintendo of America in the early 2000's before they released the Wii and were still servicing all their old consoles.

So yeah, I'm curious about how functional they are, for sure.