It has historically been available, but I am not sure if that applies to all Bases nor indeed all year round. Winter would be no problem I would assume, and at bases like LPA/TFS where you only fly 10 -12 days max a month (the days being so long ) NAS are delighted if you take it, as they get the same productivity (save a few SBY's ) as they have on 100% whilst saving 20% of your salary.
Beware though, 80% paying Spanish tax only works out around €5000/mth, if I had stayed I would have gone back to the circa €7000 on 100%.
It pains me to recommend FR over the Red bunch, but the way things have gone these last 3 years or so I have no qualms about doing so (particularly if you can get a base that suits you, which seems to be back on the agenda to get people in the door)
In short, no base stability, no guarantee you won't spend a Winter at home earning nowt, and constant deterioration in even simple stuff like SBY tickets to go home for commuters. . . NAS via NAI/OSM are really losing the plot . . . as you have wisely gleaned from all the invective published above.