Yes Signal/Molly are somewhat safe if you use registration lock. If somebody else gets ahold of your number, they won't be able to see past messages anyway since they are stored on device and not in the cloud, but without registration lock they would be able to see future messages intended for you, and you cannot restore access for yourself.
Just generally make sure that you know what you want to achieve. A number you don't control has the advantage of not getting ahold of your hardware identifiers or geolocation like any anonymous SIM card would, but you need to make sure to have forward privacy set up and a backup solution to inform your contacts in case you lose access to the account. And obviously don't store any compromising information if you use it on WhatsApp, Google etc.