It's also possible to fire people, and it has been made easier recently. The company has to provide typically 3 months of advance notice. But it's perfectly possible to just pay this and ask the employee to leave immediately (what we typically do where I work: if there's a problem with someone there's no point lengthening the issue). It's typical to pay a bit more, like 6 months severance. Bigger companies often go higher for old employees, 1 year is not unheard of. But for a start-up I would assume it's on the low end. The issue on firing is not that it cannot be done, but that there is significant legal uncertainty around it: the employee can turn against the employer and challenge the decision or severance amount. Some changes have been done to bring clarity there, I haven't followed the details. It's a true concern, but in high tech which is a small world with usually reasonable people, this is quite rare. I've had contacts with ex-employees working and I expect that to be typical. Everyone benefit by making this as unpleasant as possible (and it's never pleasant). The situation is very different when a manufacturing plant is closed, it can becomes messy with factories being occupied and that's what people tend to hear about. But when talking start-up the context is different.
There are many things to improve in France and many legitimate criticisms on the legal and administrative situation. Still I feel that some paint a blacker picture to justify their own unrelated problems, or having left the country (which is ok in itself: go and explore. No need to pretend you just left hell thought).