Electronic tachs and governor - need electricity from somewhere. Either Alt or Bat. That’s it.
Bell 47, with total electrical failure. Lose radio/ICS, strobe light, the ability to turn the hydraulic’s off, and the starter motor.
Depending where you are, you might not notice till you try and restart it.
I fly a twin turbine computer now. Autopilot fails the test a few times? - Shut it down, and “restart” it - painful. (Both engines off, all power off - almost expect an “Apple” symbol somewhere on the restart......)
That’s why they call them the GOOD old days....
Although I love the “synthetic vision”