While it's a more secure platform for messages and calls, Signal's job isn't to prevent unknown users from contacting you, so you run into the same issues you would with a regular phone number. People you know that you don't want to talk to may try to strike up a conversation and you'll still get spam messages. Luckily, it's easy to block these suckers.
Signal Private Messenger is famous for its end-to-end encryption, but you can't even use the messaging app without having a real phone number attached to it. Luckily, there is a way around this limitation, so you can use Signal even if you don't have a valid SIM card in your smartphone.