Bela looks really cool. I want one, but once the kickstarter ended it became vaporware (although they do seem to indicate it will be available to general public soon:
http://bela.io/)
For the short-term the Raspberry Pi is not a bad platform. I just got a chorus/flange/delay effect (currently text-file configured) working on a Pi3 with a Behringer UCA-202. With the OS stripped down to the minimum I'm getting 1.3 ms latency + maybe 1ms latency added from the AD/DA conversions on the USB audio interface for a total 2.5ms latency.
That is equivalent to standing 2 feet further from your amp, so it's a more-than-adequate development platform. But if you're a latency snob, or mixing the output of this with dry signal then Bela can get you down to about 2 or 3 samples latency if you simply use Bela SAR ADC & DAC (the CODEC still adds ~1ms because of the digital anti-aliasing and reconstruction filters).
The advantage of having a Linux OS underneath (as with Bela) is that all the work is done in the realm of drivers and ready-to-play access to USB-midi devices so your control surface isn't required to be a DIY project if you don't want to go that route.
At the same time, there are libraries in the PI for talking to I2C devices, so you could put an 8-channel ADC on a pair of I/O pins to scan pots or expression pedals similar to what the Bela platform does. It's just that the Bela project has already done this boring work.
Whether you go with the Pi or Bela (or something else) I suggest writing your code in a manner that is not platform-specific so you can move it around. You can also develop it on a PC and experiment even before you have decided on a specific DSP development platform. If you are leaning toward a DSP chip like a SHARC or even trying things on a microcontroller then you are safest sticking to plain no-frills C so you can easily build it into its final hardware target platform. By "no-frills" I mean don't use operating-specific functions in the DSP core functions (for example don't insert code for reading/writing an audio interface into the main DSP loop).
For example, the way my code is written it is easy to do either of these development/testing methods:
#1 I have in my files a rudimentary tool to read and write a wav file. I call my "process" function from my wav reading function and process the files. Output from processing is stored back to a wav file so I can listen to it.
#2 I use Linux and Jack Audio Connection Kit. I have a basic Jack client from which I can call the same "process" function and start it as a real-time effect. This model applies directly to the raspberry pi platform after I have evaluated it on the PC. Just copy source code over to the Pi using SCP on the Ethernet cable, compile and run. Gives me fun and joy until such time Bela becomes available for purchase.
My particular "first" effect on the Pi is a further evolution of this:
http://cackleberrypines.net/transmogrifox/src/cgi_chorus/csource/ Which in the application you see is run as a cgi program allowing you to experiment with envelope control on parameters:
http://cackleberrypines.net/transmogrifox/src/cgi_chorus/chorus.html
