Recent debate in my club : replace the Lycoming of a C172 for 15.000 euros or change it for a modern Thielert/Diesel for 30.000 euros.
Diesel Advantages : uses diesel fuel / jet A1 at less than half the price of Avgas, slighly less consumtion as well. FADEC/ single throttle management.
Disdavantages : no-ne knows the real potential of the engine 1000- 2000 Hours ? Electric failure and the engine stops ( it happens ) , Various reliability problems , recent change of engine block , no retrofit . when using Diesel, temparature limitations. ( but not with Jet A1) Weight limitations ( with full tanks the 172 becomes a 3 seater, etc...
This combined to the fact that Lycomings are very, very reliable ( after they pass the first 100 hours ) you can see why many are still replacing Lycomings instead of going for far newer technolocy engines.
But one day, someone will come with a good new engine , as 20l/h of leaded fuel per 100 HP is a bit old fashionned in the 21st century.
But reliability is what you want above all in a single engine.