The Diamond aircraft are reasonably good aircraft. I am involved with the operation of 14 DA 40 and 3 DA 42 all fitted with Theilert/Centurion 2.0 engines.
I have heard of corrosion problems in UK but maybe this is due to aircraft sitting in the open. All our flights are over the sea and corrosion is a minor issue mainly related to unprotected bolts. We do fly such high hours that our aircraft are in for maintenance every month, maybe this keeps any corrosion at bay.
We find that the TAE 2.0 is very reliable, and I do not understand aircraft being grounded by gearbox limits etc. We operate on the opposite side of the planet from Theilert and with careful planning almost never have to wait for spares.
The 1.7 had a number of issues, some to do with theilert and some to do with the Diamond installation. Since the introduction of the 2.0 almost all issues have been resolved. Recently there was an issue with clutches which was caused by faulty parts supplied to TAE. TAE responded with emergency AD's to replace the faulty parts.
This year we have averaged in excess of 1,400 hrs a month all with Diamond aircraft fitted with TAE 2.0 engines, we find them relatively easy to maintain and have few defects, certainly no more of an issue than a lycoming. Due to the high hours we fly our engineers are probably some of the most experienced with the Diamond/Theilert aircraft, maybe this has an impact on the low number of problems we have compared to those reported by other operators.
One thing I have found when reveiwing defects is that many are caused by incorrect operation or pilots not understanding the aircraft systems.