I too fit only Vaillant & am an approved installer. They are by no means perfect but are a lot better than many others on the market, especially WB in my opinion. The problems I have experienced over the years are :-
1. Combustion chamber gasket obviously but all should have been replaced on recall.
2. Internal rubber hoses as mentioned earlier - Boilers now fitted with solid copper pipes instead. More a problem with the clips IMO & I have replaced a few clips with ordinary jubilee clips. Rubber hoses themselves are not a problem, all cars use them in much harsher environments.
3. Error F75 on older boilers due to poor positioning of pressure sensor that needs to read a 0.2 bar increase in pressure when pump starts. Newer boilers don't seem to have this sensor but it is a pain in the arse on some older ones. In there wisdom Vaillant refuce to accept there is a problem but will happily sell you a conversion kit for ÂŁ100+
4. Poor design of PRV pipe which, if not careful can interfere with the removal of the condensate trap.
5. Over-complicated controls, especially BUS controls like the VRC430 weather compensator. Easier to use aftermarket such as Honeywell
So all in all, not perfect but better than most.
Vaillant are launching a "new" range in March which, as far as I understand is not much different to the current range but with all the niggles ironed out, & probably a few ÂŁÂŁÂŁ added to the cost
The Intergas boiler sounds interesting but not mainstream enough for me at the moment, both for spares or ease of selling a "name" to custards.