A310 composite vertical tails have been around for some 34 years...
However, structural design of A380 and A350 vertical tails is completely different, so there are lessons learned as well.
Composites gliders are around for some 63 years now and CFRP gliders for 46 years. Lessons with respect to hot and wet have been learned.
This however is covered today by cupon testing in the laboratory, full scale tests are done in "as procduced" (= fully dry after curing above 100°C) and room temperatur, hence an "environmental knockdown factor" is applied. Epoxy resin absorbs a few percent moisture in service.
It would be interesting to know, how many design changes are between the 787 and the 777X wing, I assume Boeing has meanwhile learned something about the 787 design as well...
It is amazing how the aviation world has changed, aircraft like the A380 and 787 were developed "on the internet", both manufacturers published an awful lot on the web, you could follow the complete design and testing period. Topics like GLARE or would fibre barrels were discussed in scientific conferences and publications. For A350 and 777X there is almost total silence, you just see some high level marketing publications and that is it.