They are solid but there are a few things to be aware of
The actuators on the turbos can stick leaving no boost. Doesn't cause damage and they are replaced under warranty. Almost all 335ds get this at some stage. Mine were done and the vacuum pipes as I pushed for them to be done under warranty.
The red boost pipe can leak mine never has again a simple fix
As with all diesels the dpf can clog and fail early if you don't monitor temps. If temps are ok they can last 120k
If the engine runs too cold they can fail after 50k
I've monitored temps for the last 2 years and I've noticed them fall from 86, to 83 to 80. This happens as the thermostats gradually stick open letting too much coolant to flow. Again not damaging but if the temps fall below 78c the dpf won't purge, clog up and fail eventually/early.
I changed mine before this 78c cut off point so try to check running temp via hidden menu. Mine were done at Darren wood for 220 quid. Now runs at 91c solid instead of 80 and they even tested my dpf back pressure for me and its only 4mB at idle and the upper tolerance is 20mB - so good running temps and you will be fine.
No other issues I'm aware of that are common.