I've been down this path. Getting people to switch was a mixed bag. Some did and have stuck around using it. But others either refused ("another messaging app?!") or wanted to but ran into problems using Signal.

I've also run into problems connecting with people on Signal. They just aren't findable, even with their privacy settings relaxed and with me trying several formats of their number. After troubleshooting and having other people try, it seems random and makes no sense.

So the three biggest downsides of Signal are: a) users having to juggle yet another messaging app (because their friends/family are scattered across several and they find this overwhelming already); b) Signal looking a bit 'ugly' of not as useable to some people; c) people getting frustrated at not being able to connect with friends who are already on there.

The biggest upsides to Signal are a) it's not Meta or some other corporation trying to profit from exploring its users; b) it's a clean and simple UX (IMO) that doesn't try to do too much extra; c) the organization behind it takes security very seriously.