They should always state the amps the motor pulls and the voltages it needs. Then you know the watt rating of that motor. Which is how powerful it is supposed to be by design.

The KV only really suggests the application or whether it's going to be a small prop, high velocity power plant (that might need a gearbox) or the other kind.

I'd say that more often than not, they do provide the amp ratings. And almost all give the voltages to provide. So very often you have a quick way to work out the power, which is what watts is.

My guess is the reason so many model motor advertisements and spec sheets are so worthless is the source. They really haven't a clue what we need to know, and simply don't care to look into our hobby to figure out what would provide better service, because they've not had to. Until one of them pulls it all together and starts cornering the market, they'll just keep on keepin' on.