Arduino is a very popular platform for novice programmers to get up and running with audio projects without worry about the hard part of dealing with configuring the peripherals on a microcontroller.
The Teensy boards are compatible with Arduino and offer much faster performance. The most powerful board is the Teensy 3.6, it's 180 MHz with real hardware floating point support. This is actually powerful enough to experiment with decent effects, even frequency domain pitch shifting, etc.
The problem with the shields for these various arduino boards is they operate at line level, and usually from 3.3V. As soon as you want to plug a guitar in, there is a host of issues. Guitars need very high impedance inputs otherwise the pickups get loaded. Typically 500K to 1M, but line level is usually 10K. There is a big issue with signal levels. Line line level is usually around 1vpp, but guitar signals can be as low as tens of millivolts for vintage low-output pickups, several volts for active pickups, and if you put a regular pedal in front of it, it could get up near 9vpp depending on the circuit design.
Finally, the boards always seem to have 3.5mm jacks, not 1/4" jacks.
Ultimately, the requirement for a digital effect for use with guitar is a 9V powered input buffer with high impedance, with appropriate circuitry to either boost the signal up to line-level, or attenuate the signal down to line level to ensure the ADC isn't damaged.
I've designed my own board to meet all these requirements featuing a WM8731 codec.
The technical discussions that went into the design can be found on the Teensy forum here:
https://forum.pjrc.com/threads/42678-Teeny-Guitar-Audio-Boardand the schematic is in post #46
https://forum.pjrc.com/threads/42678-Teeny-Guitar-Audio-Board?p=137884&viewfull=1#post137884Here's some renderings of the board. The gerbers left for the board shop today!



Find more Guitar DSP and modelling info at my website:
www.blackaddr.com