Zipstiks are easy to open and so is replacing the microswitches. So if you have any spare ones, or can maybe cannibalise some from a poorer stick, they are quite easy to fix. Failing that you can open the broken microswitch itself, it usually is just a stuck spring that is the problem.
I have a couple of Competition Pros for the Amiga, and have a USB version for the PC. I really liked them just for that reason that they are easy to fix when something goes wrong. Having messed around with both, it seems the only real difference between the Zipstik and the Competition Pro apart from the look is the switches for the firebuttons. While the Zipstik had proper microswithces for them too, the Competition Pro had a pair of long connectors that bent and connected when you pressed the button. These would get worn and not connect any more after some use, so as they got older I had to more and more frequently open my joysticks to bend these a little so they would connect again.
The new USB Competition Pro however has also proper microswitches for firebuttons and is not using that solution the old ones did. So while it looks like a Competition Pro, it's actually built like a Zipstik. So it seems to me it's some sort of hybrid of the two.