The Thielert engines are indeed not very reliable.
Many schools bought it in a bid to reduce operating costs but often costs were found to be higher than operating a piston engine twin because the plane was more often in the hangar than in the air.
At the overhaul, they need to remove the engine and put a "new engine".
I can't tell for sure and many customers surely don't know whether these are really factory new engines or just reassembled engines, because Thielert does it all.