As reliable as anything else modern but not super fancy, ie very. TTRS will be much more expensive to run because of the RS tax, but ultimately a much better car plus that engine is worth it.
Any car can be a possible lemon, I’ve no doubt there’s plenty of Zeds that have turned themselves off for no reason too. Flukes happen, it’s the known issues you need to worry about. Check the owners forums for more detail on those.