Did I understand this correctly? Crossbows reload slower when it's raining?
If this is true, and the reason for that is that crossbows work less well in rain, then I have to say this feature is both unfair and "wrong".
It is unfair because no other weapon gets influenced by weather, so why punishing a certain class?
And it is wrong because crossbows DO fire worse with a wet string, but so do bows. The only difference is, that you can transport bows in rain without problems, as you could unspan (?) them and carry the string under your helmet, where it was kept dry. But you couldn't unspan a crossbow, you probably need a working bench for this.
But as soon as you have the bow spanned and it's raining, the string will become wet, too, and so the bow will suffer, as well.
If also bows suffer from rain, the "wrong" argument is to be ignored, if there is no such feature at all, just forget what I wrote. But if only crossbows really suffer from rain, I would ask myself why in this case realism goes over balance.