BRIEF: the problem is not that it is a startup, the problem is the lack of seriousness. They are just born and they behave like a consultant company with 20 years of experience, trying to fool you in everything
LONG: I passed their interview. In a short time I met them again to discuss about my situation (with their chairman). The chairman is a businessman who is trying to rule the company as an old, famous, 100% sure company, but not giving the same guarantees: 1. the contract is "a forfait", with 228 days (instead of 218 as per collective agreement), meaning that they can force you working more than 8 hour per day, and even some weekends, 2. they propose stock options, toilet paper if the company fails, to be kept at least 3 years (meaning that if you resign you loose everything, as if they fail), 3. no guarantees of salary increase, as other local companies do (ARM, Amadeus, Intel...), 4. no changes to their offer even if I had a better one.
I refused for the first time.
To make me change my mind they negotiated everything. But then they started taking time: I am almost sure they tried to create problems with the other safe, reliable, good contract I have. Indeed at one point, bored about useless waiting, I sent the other contract to the other company. They took almost one month before sending a "lettre d'embauche" BY MAIL, without their sign and without lot of details we had decided togheter, but with the working day come back to 228 without any discussion about it.
When I went to their office they convinced me to sign anyway. But the same day they asked to send back signed the lettre d'embauche. Signed by me, not by them. That would have engaged me, forcing me to accept any contract, even with nothing of the agreement we took, while without their sign they could have always said no. And I should have signed it with another signed contract already sent. I said no. Either they would have sent their contract in few days, or I would have not accepted.
They disappeared. Their strategy was to fool me to have the contract that they want by playing on timings. They are not correct at all, and not serious. Be aware