Avantages
It's a pretty horizontal company (barely any middle management). The company is usually very focused on educating junior developers and sharing knowledge through communities of practice, where participation is encouraged by keeping them within your workday. And a high focus on delivering quality overall. Whenever there are conflicts with customers, they're always willing to provide you with advice and side with you, especially if you've kept open and transparent about any issues.
Inconvénients
The company is way too dependent on one big client. However, it is pretty stable. Besides that, it has survived multiple vendor cleanups post-pandemic. I'd bet mostly by the focus on quality they put on their work.