The Thielert also has maintenance drawbacks. We checked
into getting some for our 172s, and to maintain them you need either
to take them to an approved repair facility, or go to Texas and take a
two-week (IIRC) course in their maintenance.

A new engine needs a new skill set, simple as that. You wouldn't give
your car to a mechanic who has only repaired Ford T engines so far and
have him check the electronics, either!

There's no TBO. They call it a TBR, where the whole engine is

And where's the disadvantage? You know exactly what it will cost, and
Thielert even refunds you pro rata, should the the engine not make it to
TBR. A fair deal, I would say.