The Blitz R-VIT/HKS Camp II unit takes readings from the OBDII port and can display variable information including boost.

My question is how accurate is this boost signal coming from the OBDII port compared to an ordinary boost gauge where you take reading from the vacuum hose?

Does it mean that it would only read the correct boost level only if the ECU is used to control boost? If you used a EBC/MBC or put in an upgraded actuator in the car, the OBDII would not taking the readings accurately?

Thanks.