my experience with transistors was, that they are also not very accurate, especially when used not as a switch (open/close) but as a voltage "following" amplifier. did you get consistent results with a transistor setup that way?
this can be a blessing and a curse i guess... and it is not necessarily true either. there are very fast vactrols around, that can detect very fast frequency changes. (so fast indeed, that you can introduce 12khz whine with a PWMd LED for example) i assumed also that all they were not very fast and once got a very lengthy email from a guitar pedal guru, explaining that all in detail...