actually, a lot of companies have gotten better about that.

Anyway, back on topic, yeah, the rating of the speed controller just has to be big enough to not fry, other than that, it's the least important issue with putting together an electric system. The only reasons not to put in way over-rated controllers is price and weight.

Estimate power required based on the motor and battery, then pick a controller to match. If the motor is rated at 20 amps, anything bigger than that is good, anything less is risky.

Even though the motor is often more expensive than the controller, if the motor fails, you glide in for a landing. If the controller fails, if it's a BEC-equiped system, you may crash. Even if it's not a BEC system, it's still connected to the radio system, and could cause a loss of radio control on failure.