I thought I solved the reliability problem by installing a Huret odometer. I solved the cadence/speed problem by memorizing my gears and a range of speed-cadence relationships and doing the arithmetic in my head as I rode. The odometer was great, though I expect o-ring slippage made it less than perfect. That wasn't all that bad - I could take heart in believing I always rode farther than the odometer read.
I got a Cateye Solar in '84-'86; I still have it, and it worked until I replaced it a couple of years ago with something modern. I'm still using the magnet for sensing speed. The solar batteries are no longer made, but LR44s work.