Instruments can be unreliable!
Three times in a glider have I realised while being pulled up by the tug plane that my instruments were telling lies! Mainly because the combination was moving along nicely and gaining height; although the Air Speed indicator was reading, at first, ZERO! Don't often see zero on an ASI, usually the lowest number is the stall speed, they don't bother with other low numbers....
Though my primary attention had to be keeping correct position behind the tug, I did notice the needle was moving round steadily in the ASI, passing all the suggested airspeeds and ending up on the stop! Oops. I must have forgotten to remove the electrician's tape, usually applied at night to keep damp out of the tubes...
OK, I planned to take a higher tow, and fly around for a bit to see what it felt like. After release, and checking my surroundings for any other aircraft, I performed a series of gentle stalls. Right, know what that feels like and the stalling speed is about 30 knots. So finding rising air, carried on not too far from the airfield for half an hour or so before deciding to do a normal circuit.
At my home airfield, no problem. In Scotland, at Aboyne, they commented that the approach was a bit lively, no different than usual. In Russia (the USSR) it was their glider - a Standard Jantar, the ASI was in kilometers anyhow so I did the same as at home port, did some stalls, felt when I had good control, came in and landed. My Russian groundcrew, all three of them, came rushing over and said "TOO FAST, TOO FAST!" So I climbed out and pointed to the tapes that had not been removed.... Apologies all round!
So instruments, who needs them? A compass is useful, seldom fails, but if you know the time of day, the position of the sun may help. You should certainly recognise a stall with no ASI. Altimeter? Cows get bigger. Certainly should be able to tell by looking at the ground if you are getting too low... Likewise, following a motorway works pretty well for getting home. But in a glider, the instrument we find most useful is the audible variometer. You don't have to look at it but it tells you if you are in lift or sink. The altimeter may help, but not a lot.
In power, much more complicated, and noisy. Good to know what fuel remains, if the donkey is getting overheated, etc. Add radio to the mix and your lookout may suffer....that is after all, your primary safety device; your eyes, and the horizon. The real one.