Although Zamperla are incredibly successful and supplied several opening day attractions for Disneyland Paris, I think Zamerpla's success is more to do with the fact they're cheap rather than their quality, although I'm not necessarily saying that's why Disney used them. Disney use Vekoma quite a lot, whereas generally Vekoma aren't the favoured manufaturer for major roller coasters (their Mine Trains are very good and of course their haunted swings have done well).
Nonetheless I don't think you'll find a lot of people who work in the industry rushing to praise Zamperla for the quality of their products. In fact I've spoken to quite a few engineers from different parks and their experiences don't tend to be entirely positive. Although to be fair to Zamperla you get what you pay for and not every park can afford a Mack or a Gerstlauer etc. Generally speaking (and this is a generalisation) the Italian manufacturers are the budget manufacturers. It doesn't necessarily mean that Zamperlas are poor value for money (I suspect they're reasonable value for money of they wouldn't be so common), but I don't think they're normally a good manufacturer to go for if you've got a big budget and want something that's really well engineered and comes with a high level of service.
It's not just about the quality of the build, but also down to other factors such as how quickly they can supply spare parts when you order them, the quaity of paperwork (technical drawings etc) that comes with the ride to assist the park's engineers and general after sales service. There are quite a few factors that affect the quality of a ride that aren't immediately obvious, for example the quaity of the electrics, how the PLC works etc.
If the park has a good relationship with Garndale and find they've been giving good after sales service then why not use them. And if it means supporting a local engineering firm in the process then all the better. At this stage we don't know how reliable or good to operate it will be, but if it does turn out to have problems I won't necessarily say that they should have gone to Zamperla.