Thielert trashed the diesel market with their stupid accounting antics which led to their bankrupcy.
They also had constant reliability issues, which is not to bad for a flying school (I am sure most DA42s were bought by FTOs) doing short trips, but is no good for a private owner doing serious flying, especially to places where there isn't much avgas which is, ahem, probably why one would buy a diesel in the first place
Time will tell, but it won't be tomorrow before one can be sure they are proven engines.
And people I know very well who do Diamond maintenance tell me the build quality is still poor, with simple stuff like corrosion of simple metal fittings within a year. Again, OK for a school...
The GA business has only itself to blame for the lack of progress.