I have almost complete faith in the totaliser. In the DA62 it’s more accurate than the pilot judging the rim of the fuel filler cap (probably within 1 or 2 litres on a 326 litre aircraft). On the PA31-350, we have a retrofitted JPI EDM system. That will be accurate to within 5-7 litres on a 896 litre aircraft.
Connect all that to your GPS and, with the correct training, there’s almost not excuse for fuel starvation.
Peter wrote:
except that the dodgy fuel systems tend to be in MEs
The “dodgiest” fuel system I have ever used was in a V-tail Bonanza. Tip tanks needing transfer to the main tanks before fuel could be used, and an aux tank that was only usable in flight, combined with a single fuel gauge and two switches (aux/main, left/right) and a fuel return line going into the left main tank only made provided hours of amusement.
The PA31 has an interesting gotcha if you have the optional nacelle tanks fitted (they bring about 1:20 – 1:30hr to the party). The tanks feed into the mains by way of an electric fuel pump. When you select the pump, you get a nice light to say that it is connected, not that it is necessarily working. If you then have to rely upon the trusty float gauges, you may be in for a surprise when you realise a pump hasn’t worked, an hour or so before you thought you would run out of fuel. I know of one pilot who spent an uncomfortable few days at an oil well support airstrip in the Middle East because of this. 
Since then we have changed our fuel tank management strategy such that we prove/disprove the feed from nacelle tanks by sticking on mains & nacelles prior to using the outers (they have about 1:45 in them).
Oh, there’s another gotcha where the nacelle tank may slowly drain into the main due to gravity. A puddle of AVGAS at morning A Check is a good indicator if you’ve been daft enough to refuel all tanks the day before.
The problem inherent with GA fuel tanks is they tend to be flat and large as apposed to thin and tall and since planes are always moving around it becomes very difficult to measure the remaining fuel as it sloshs around.
