We're looking for a hiking trip in France (Annecy, Chamonix) and Switzerland Jungfrau region in mid-May. Unfortunately it seems that May is in between seasons and not a lot of open. How much is still open and operating? Is it advisable/still worth it to go?

I would never discourage anyone from visiting Switzerland regardless of time of year.

We often visit in the off season and never run out of things to see and do (we're big hikers).

Plenty is open - alpine villages are populated with year round residents who need groceries, etc.

However, some mountain excursions may be closed for off-season maintenance, and some hotels in alpine villages will be closed, but not necessarily all of them.

It's a great time to rent an apartment, as owners are more willing to rent for less than a week and prices are lower. It's also quiet, which is very much a good thing as far as I'm concerned.

So, ask yourself what you most want to see and do - and then research those particular things - is there a particular mountain excursion you want to take and will you feel you've missed out if it's not operational? If so, check with the individual websites for the excursions and hikes you most want to take and then decide if May will work for your particular trip or not. There may still be snow and mud at high elevation, but that's not necessarily a deal breaker for hikers either.

You don't say specifically where in the Jungfrau region, but you can check some operational dates here:

https://www.jungfrau.ch/en-gb/

IN THE FRENCH ALPS, ALMOST EVERYTHING WILL BE CLOSED.
iN THE SWISS ALPS, ALMOST EVERYTHING WILL BE OPEN IF THERE LIES NO SNOW ANY LONGER.