Perhaps not the most perfect aircraft in aviation history but I doubt if such a basic bog-standard system as the Q400's hydraulics would have passed certification on both sides of the Atlantic if items like filters, and maintenance intervals, were not up to scratch. Makes you think.
They certainly had a design-and/or-maintenance problem with the actuator rod threads but again we must wait for the final report to see if there wasn't a mitigating factor. Absolutely ignorant of things like de-icing fluids but have seen suggestions (which may have been self-serving on someone's part) that this may have been cused by a particular type of fluid which reacted poorly with the u/c.