Did mine in December.
One big advantage is there is less VFR traffic around to block the radio. I can't imagine what calling Solent in the middle of a summer saturday for an approach would be like.
Yes the weather can be bad but it can in summer too. We never encountered icing problems, but often it was too windy or worse gusty. Never do your test on a gusty day.
I don't think it makes much difference when you do it.
Whenever you do do it good luck, the test is one of the most interesting moments (or 2 hours) there is.