I imagine unrestricted usernames would make spam easier than with phone numbers. I'm just hoping they have a way to control this.

Sorry I misframed it. I mean that since there's no need to reveal phone numbers, there could be an opportunity for spammers to increase spam by creating many accounts, and Signal should preemptively find ways where such spams could be reduced. However, I realized after posting that the article says we still need to sign up to signal with a phone number. However, there's still the risk of impersonation (by writing someones username with tiny changes) and people trying to add vulnerable users by username (which they might be using on other platforms) instead of phone number.