Remember the old adage 'one size fits all'... ever try that with shoes or a shirt? Yeah, one large enough will do... but do you really want to walk around in size 15 EEEEE wooden boxes; or stuff your 12D feet into 10B canvas? Or perhaps we would look more professional wearing a gunny-sack in place of our proper fitting workshirts?
With metering of refrigerant; a TXV will meter more accurately over a wide range of demands... where a piston is just a middle of the road compromise.
In my climate where temps can range over 30+ degrees; and we can be sticky humid, then rain, then sticky humid again--all in one day.... Expecting a piston to handle all those indoor climate changes... well IMO the customer gets the short end of that deal.
This is not aimed at anyone, just my opinion: If the tech staff does not understand TXV's (proper installation and proper charging)... then perhaps they need some professional training???