The heavy technology that increases efficiency of car engines works primarily at part load, where aircraft engines don't operate much. Complexity is not an asset in making avgas aero-engines work better, and diesels only make sense as a tax-dodge.
Exactly. Simple engines work fine for aircraft, if comparing efficiency.
Diesels got hit on two fronts: first by Mr Thielert's "little accounting adventure" resulting in its collapse, and second by taxation. In the UK, the financial case for an aero diesel has basically disappeared.