Problem with using a rule is you need to have the chain stretched tight and you need to line up exactly with the middle of the rivets – both things are going to give you a source of error which I like to take time to minimise. Therefore from my experience I like having a chain checker to quickly check chains – especially on friends bikes when they ask me to check something for them just before a ride, especially drivetrain problems, when I can quickly check the chain is not completely beyond any replacement point. I use the Park one so can’t comment on other ones giving false readings though. As for the rollers – it’s only going to be affected by the 2 it touches so the error is probably less than the errors in the two points I made earlier and I’d rather they tell me a chain needs checking properly than let me carry on with a worn chain.
But that’s my preference. I then use a set of calipers to measure the chain properly if in any doubt.