Signal is vulnerable to MITM attack. Our government has no problems with reading of my messages in Signal. I forced either to accept new certificate and be OK with wiretaping, or to lost communication channel with a friend.
The number of countries where the government would do that to your SMS but won't also just arrest you arbitrarily and seize your device and/or beat/threaten you, seems small. But I imagine there are some people in some places, who feel physically and/or legally secure from arbitrary government action against their devices proper, but not with their cellular service.
So in other words, such attacks aren't viable for dragnet surveillance and must be targeted. But if you have reason to believe that you'll be targeted by the government for surveillance, you'd be on the lookout for signs like "your safety number has changed"?