System failures like tachos are not peculiar to R22s of course. In fact, the inbuilt engineering redundancy in R22 RRPM control aids is very high so you will be unlucky not to get some help in RRPM control - (there are 5 separate aircraft systems).
When I did the Bell 206 factory course (which is unbeatable if you fly their aircraft) they made mention of a number of "engine failures" where a perfectly serviceable helicoper was flown into the ground because of a tacho generator failure giving a spurious indication. One of my colleagues had a torque indication failure with other failure indications in an AS355N just a couple of days ago. It all ended uneventfully, fortunately.
Experience can certainly help in these cases, but experience is expensive and difficult to obtain. However, a small part of all that time we spend on the ground waiting to go flying can be usefully spent with the AFM and technical manuals getting to understand how these things work. When something does fail you are in much better shape to intelligently assess the problem. And best of all, you can gain such knowledge at NO COST!