I assume you are saying your tire guy doesn't think the valve stem extenders work that well, and he is not saying that a TPMS doesn't work that well.
I have also read some folks have had problems with slow leaks when using metal valve stem extensions. I am putting my TPMS on this week. It has the flow-thru sensors so for now I am not going to put is on my inside duals, just the outside ones, front tires, and two trailer tires. When I have a new set of tires put on in a couple of years I will have long metal stems put on the inside duals then.
I agree if you do buy some though they should be solid metal. My sensors are big enough that they would probably cause the valve stem to flex while under way if made of flexible material or rubber.
Full Time in 2017 Newmar Ventana 4369
pulling a 24' enclosed (Mini Cooper/Harley/Kayak)
(traded 2014 Thor Challenger)
US Army 70-73 Retired LEO