lovefromisrael but what else are the disadvantages cmopared to let's say.. whatapp?
Other than what @[deleted] mentioned regarding the user base, which is obviously a major consideration, I don't think there are really any disadvantages. At least from a security/privacy perspective, I think Signal is just objectively better than Whatsapp.
The biggest issue with Whatsapp is that it's not open source. So even though they claim messages are E2EE, you really have to just trust them.
I'm sure you could argue about UI design or reliability of notifications, or extra nice-to-have features. But if you care at all about security and privacy, then there is really no question that you should prefer Signal over Whatsapp (except for that pesky problem of getting the people you communicate with to use it).