I have experience with using the AudioInjector Zero for guitar processing. It works great at long as you have a preamp or unity gain buffer feeding the input. I created an open-source project around it:
http://treefallsound.com
I tried many different op-amps (5) and discrete transistor buffers (2). The biggest problem I had was noisy power since I needed it to be powered from the same supply as the Raspberry Pi. The 5v rail is simply unusable without regulation. The 3.3v pi rail is regulated and less noisy, but still not usable IMO. If you could steal 3.3v from the AudioInjector, that would probably be your easiest option. That would have required a hack to the Zero card, so I settled on regulating the 5v rail with a AMS1117-3.3 low-dropout regulator (same regulator the AudioInjector Zero uses).
For the buffer itself, I found the MCP6292-E/P opamp (
https://www.mouser.com/ProductDetail/579-MCP6292-E-P) to be the best performing. Best noise floor and output swing. You'll want a rail-to-rail opamp cuz even 3 volts is not much headroom for a guitar signal. If I could justify the extra cost and board footprint for my project, I'd go with their "isolated soundcard" which has almost double the headroom and I'm sure the noise and audio quality are much better too.
If you'd like a schematic for the opamp buffer and supply regulation I use for pi-Stomp, let me know.
Cheers,
Rand