I mentioned only stats for 2017. Fiesta had pretty good numbers (not that good in 2014 when it was new).
I don't think that the age of the rally car makes it worse in relation to reliability. That is more about particular team preparation and maintenance and I think that we can assume that all cars get in average similar maintenance. How old is the bodyshell is imho not very important (see Loeb's Xsara WRC stats).
On the other hand we may ask whether older cars actually aren't generally more reliable because their technical solutions are proven. The new cars often tend to be unreliable because some issues are very hard to find and solve during testing.