I mentioned earlier here that Speedhut says that it isn't waterproof. So some sealing would be required. I was thinking in terms of clear silicone RVT but getting a decent seal around the lens might be challenging if the tach can't be disassembled. Vibration might also be an issue but my GSXR has a stepper motor tach and works fine. It will come down to how well the unit is built and I'm not a salesman for them.
Instead of switches, the pulses per revolution setting is set electronically.
Those cheap digital tachs commonly rely on an inductive lead that is a wire wrapped around the coil wire (see the pics here
). They can give erratic results due to varying spark intensity on the wasted spark. I'm fairly sure that's what sharpy was seeing in the #103492 post here
. Near the end of the first link I posted some results of my fiddling around that didn't make that much sense. I now believe that my timing light's inductive lead was missing the weaker wasted spark.