Depends on where you live I'd like to think, they probably last longer and have more support for example in Europe than the US

Asking if an engine made by an entire nation are this or that is never a good idea - I'm sure there's quite a few different ones and there's always A LOT of variance.

If you have a question on a specific engine - feel free to specify.

I'm assuming you're talking about the popular 1.9 MultiJet Diesel engine (Fiat /Lancia /Alpha / Suzuki/ GM / Opel / SAAB) - The world's first common-rail in a passenger car. They are considered bulletproof, but the latest versions with 16-valves have a slightly mixed reputations.

Another popular Italian diesel is the 1.3 MultiJet and here they're known to need timing chains, burn tons of oil, dilute oil, have sensor issues, carbon up the intake and in typical GM-Opel style have plenty of PVC issues. If you live in an area that mechanics are familiar with this "creation" than it's not that bad, it's a very efficient engine.

(I've seen claims of 4.0 - 3.5 L/100KM on cars like the Fiat Tipo 1.3 JTD and Opel Crosa CDTi - but I wouldn't expect to get that kind of efficiency from them - from my experience, these never reach the stated economy!)

Generally, engines can be good - but I can't think of a good car they were installed in, There's not a single car I'd even consider.

MAYBE the Suzuki Swift 2010-up has the 1.3 diesel, and it's a good combination - but SLOW, economy is MEH, it's rare in most parts of the world. BUT honestly, it's must more fun with a Japanese gasoline M16A / K14C. and the popular 1.2L gasoline variant is super reliable and efficient anyway.

One of my cousins had a Fiat Linea Diesel in India. Back then in 2008, the marketing was high for the car, therefore they had got it.

And as Dan has advised, the car gave an amazing mileage at 5.0 -5.5/100 km in city and 4.0- 4.5 in highways. However, again they required regular maintenance. The dealer did not know what they were doing and once when he had a major sensor go out, he had to wait 28 days for the part to get shipped to the dealer. 

Expensive maintenance was a pain. 

The engine noise was louder than his Honda civic 2001. Eventually he gave up this car and went for a Hyundai I20 diesel as they have better sales support in India compared to Fiat.