Hundreds of French (and other European) sailors cross Biscay every year IN THE SUMMER without mishap from or to every marina on both coasts. Weather forecast
is reliable. But don't do it during or just after a gale...Remember that there is NO port of refuge beyond St-Jean-de-Luz (almost on the French Spanish border) north to (possibly) Royan, and (much safer) La Rochelle in case of bad weather (including high swell). The Arcachon Pass is impossible to cross in bad weather There is also a large bombing / missile firing training zone N of Arcachon. Give it a wide berth. Stay largely offshore
for the main part of the trip, at least up to the Gironde river mouth. Note the VERY dangerous area on the northern shore of that river mouth, and do not attempt going inside Ile d'Oleron on a first visit.
The (expensive) Imray nautical guides are well worth having.