I wouldn't trust it unless I skimmed through the code myself or consulted some trustworthy code experts of some kind about it, or it had a lot of battle testing and opportunity to be exposed as fraudulent or unsafe and hadn't yet.

You want to have something with a lot of critical eyes on it or with your critical eyes on it, before you put a private key in it. Exclusive use of dedicated signing software like the Alby extension makes nostr much safer. It's like the separation of entropy offered by hardware wallets in the bitcoin world, just applied through separate software applications. This protects your secret keys.