Until you check them all agin' yer cheap Chinese digital calipers, you'll never know which is right...
I had to dig out -- while on vacation, so selection is limited -- my rulers / measuring tape / not-cheap not-chinese calipers and compare them.
A Bundeswehr surplus steel ruler, a "Co-Tech" China steel ruler, the 6" ruler from a iFixit "Professional Toolkit", a vintage "Mako by Stålex" steel caliper, analog, a Hultafors measuring tape (EU Class II, so cheap) and a Hultafors EU Class III folding wooden meterstick were extracted from the toolcase.
The steel rulers and the caliper agree that the iFixit one is crap. I estimate it to be off by 0,4-0,6mm over 150mm. The others are impossible to gauge any diff by eye, so are deemed to be in agreement. The measuring tape and the folding stick are off a bit less than the iFixit one, with the tape holding a slight edge to the stick. The stick, across one joint, is off by a guesstimated 0,3mm over 300mm against one of the good steel rulers. It being EU class III this is within the specifications. Not bad for
a piecetwo pieces of wood. The tape is a advertising one, so not the full professional quality, only quite good. Tape is made in the UK!
Conclusions?
No surprises. Steel rulers often are the best alternative if you can't measure with a caliper. I do not carry any plastic ones in my kit so can't compare with them. Tapes come in various qualities -- I've got a "Talmeter" at home, which is not a marketing product, so will try to remember checking it too. Somewhere I've got a Pentair / Schroff HE/TE steel tape, and also a Knürr one.
The iFixit one being crap is not surprising either.
Tape and stick are for carpenters. Their desired precision "decade" is "one floor, one foot, one inch" so a basic error of 0,1% is not a problem for them.
Measurement -- it's a rabbit hole lined with broken hopes, products and assumptions in so many ways.