From what I can see the engine has problems when the instalation has not been built by Thielert.
The Thielert retrofits seem to be more reliable but short on power for the PA28 and to a lesser extent the C172, The Robin DR400 instalation seems to be the best but Robin's did not overstep the mark and initaly only put the engine in the lower power trainer market.
With the introduction of the 155HP engine Robin now have the power to replace the 180HP lycoming with a diesel engine that excceds the performance of the Lycoming (if the numbers fron Robin are correct).
The only issue to be sorted now is reliability and I think that the factory are doing quite well with the instalations that they have built, however it is still a very close run thing between the costs of running the Avgas and deisel engine.