They aren't unfixable. It's just that they are so plentiful and the only value is the PPU. If that is toast it is an expensive cost. Only real pain is that sometimes you get nasty battery damage with them. At this point it is almost cheaper to buy a complete board already populated than fix a broken one that needs a PPU. It's a huge shame and I hate that it is this way. They are also kind of a hassle to troubleshoot on the bench due to the inverted video and audio amp requirements.
Vs system boards are fairly straightforward to troubleshoot, especially if you know the CPU and PPU is fine. Once clocks, CPU, and PPU are verified, I've always moved to RAM and ROM access with a known good romset.