The amount of backpressure from any CSV is determined by the size of pump you have. Average backpressure is between 90 and 150 PSI. All backpressure does is make your pump think it is in a deeper well, so it can pump less water when you need it to do that. Even 150 PSI of backpressure is no different for the pipe than setting the pump at 340’ deep, which is very common for pumps. Even 160# poly pipe can easily handle this, so any other kind of pipe that would normally be used down the well will have no problem.
Backpressure is something that needs to be addressed when using a CSV, especially if the pump is greatly oversized. Even then the backpressure will only be a concern if your pump is designed for setting at 500’ or 700’, as with something like a 10 GPM, 3HP.
It is not unusual for a pump guy to be confused or concerned if he/she has never used a CSV. However, this is just his/her first step into becoming educated on the subject, and learning a better way to install pump systems. As you said, research does not show any problems with the CSV. Being this is our 19th year in business, that is saying a lot.
The pump people you need to worry about are the ones who say a CSV doesn’t work, or that they love the Variable Speed Pump systems, also known as VSP, VFD, or just “Constant Pressure Pumps”. These people are either greatly behind in the learning curve, or just blatantly out to get as much of your money as they can, as often as they can. People who sell pumps don’t love the variable speed type pumps because they last a long time and save the homeowner money. Just the opposite is true. A little research will prove this as well.