Looks interesting! Multiple certifications (2020), no blockchain technobabble, open source. Downsides: I always get worried when apps are sold as "game changing" or the most secure or whatever, as opposed to something more nuanced. Also, iOS app has not reached v1.0 yet, and from what I can tell it doesn't have enough reviews to display them. It's based out of France - I personally have no idea about what that entails in terms of legal privacy protections. From what I can tell, it doesn't even have an English wikipedia page yet.
A clear downside of this being new-ish tech is that there has been no test of time for this. With e.g. Signal we know what the results is if governments demand they hand over data: what they store is basically date of signup and date of latest use. With this, and any new messenger app, we basically have no idea. For all I know, this could be a new version of Operation Trojan Shield/ANOM https://en.wikipedia.org/wiki/ANOM. We know that French police were very involved in e.g. the Encrochat thing, so I think that there are reasons to be somewhat skeptical of this thing.
Related to the above, another downside of this having a small user-base is that it makes government attacks less costly. Attacking e.g. Signal would mean going after journalists and political activists and everyday people on a global scale - so many people being on it means the sheer amount of data entails some type of technical protection, along with the protection provided by political backlash. Going after a small number of very privacy-conscious users is less risky and less noisy from a government perspective.