That's not easy to answer.
There aren't that many to compare with the 1.2 FIRE and TA's, yet!
They're a bit too new to throw up faults on the forum as well. Cars with faults will be still under warranty, when they aren't the forum will fill up with "what the f**ks wrong with my car?" posts.
Like your write, there is some new tech but there's also a few less life limited parts as well, there's no turbo on the 500/Panda version for a start.
I seem to think the base engine (FireFly) has been around for a while. It launched in 2016 in South America first in the Uno, Mobi and Argo though the latest version in the 500/Panda here in Europe have direct injection rather than indirect of the earlier engines.
The MHEV is fairly basic, a battery and motor/generator similar to a large alternator that helps power the car and recovers waste engery under braking, it's operation will be via software and sensors.
Plenty of current cars use similar systems without much trouble as it's open technology unlike say Toyota's hybrid Synergy Drive system which they tend to keep for themselves and works quite differently.
Hybrid cars tend to be less stressed engine wise as the ICE isn't doing 100% of the work all the time as when the engine is least efficent (and working hard) the electric motor helps with the load so it's no longer working as hard.
I also think these will use a modified Atkinson cycle (Miller cycle) under low loads which tend to be less stressy to the engine as it reduces pumping losses. (load created by the engine "sucking" on closed inlet valves on the over run).
The cam is ran via a chain, so no cam belt or disasterous Bio belt to worry about.
So I would say it has a good chance of being pretty reliable if looked after correctly.
Sure, there's always a chance things will fail like any sensors, they also still have wearable parts like clutches though you'd think the brakes would last far longer if it's using the hybrid system to capture this kinetic energy.
You also need to factor in some of the other 500 foibles that I reckon Fiat haven't addressed, like the fragile tailgate wireing loom and the single use door handles!