Spent a couple of good holidays there; just a couple of quick points:

yes it does rain a lot. In some ways it's part of its attraction.

the rental season is quite short compared with eastern/southern Spain - it doesn't extend much beyond July and August

In my experience, rentals in northern Spain and particularly Asturias generate a lot of interest.

I cannot really comment on the length of season but potential customers certainly show a good interest through most of the year.

The properties concerned are I believe all country properties (no apartments) and appear to be able to command a good level of rent.

Hope this is of help