The simple reason David is that some acrylic primers neither adhere very well to both metal and plastic or are easy to sand. I've only used two acrylic primers before having the luxury of being able to safely use lacquer-based ones and had mixed results with both. The first was Vallejo and it is, as you say, possible to get a decent finish, but you would have to be extremely lucky to have that happen on a first try. Especially without a decent thinning ratio or without retarder. Even if you were fortunate to get a decent finish, sanding it back is virtually impossible. As already mentioned, it tears and will not feather down. The second was Mission Models and I just never ever achieved a consistent finish.
So, what do you do? You move onto something that does work for you.... simples