You mentioned quality ... but even the cheap ones merely count wheel rotations too, which is a simple task that even the cheapest cyclecomputer can do.

However, there are a few quality related things that can make a cyclocomputer less accurate --

1. the magnet/sensor setup may not catch every revolution. This is especially likely if they're not installed properly -- if the magnet is too far from the sensor, stuff like that. But the better quality ones often have better magnets and more sensitive sensors and so are more tolerant of bad installations.

In some cases, some percentage of revolutions may be missed when going extra fast but things will be fine when going slower.

Corrosion in the contacts between the unit and it's base could lead to a situation where some revolutions are missed as well. The better cyclcomputers could try harder to keep water out of there or use a metal that doesn't corrode.

2. the magnet/sensor setup may catch extra revolutions. This is unlikely for wired setups (though not impossible -- I can see a bump being detected as a rotation in some setups and cases) but quite likely for wireless setups.

A cheap sensor may "vibrate" occasionally and pick up one pass of the magnet as two revolution.

Some wireless computers really freak out when they pick up noise, saying you're going 99 mph or something along those lines. (And it could go the other way as well, interference making your bike seem slower, especially on the better ones that try to code their signals so they only pick up their sensor.)

Really, if you want it as accurate as possible, you probably don't want it to be wireless (unless it's a GPS based unit, of course.) Some may work perfectly in most situations, but eventually they too could run into something they couldn't handle.