Theoretically: fuck no. Private operating system, not open source code, installation source is Apple itself, Apple helps law enforcement, Prism scooped up all unencrypted data.

Practically: signal wouldn’t keep developing an iOS app if there was hard evidence Apple couldn’t be trusted, Apple gives plenty of options for your data to not be collected in plain text and given to law enforcement (turn off iCloud backups, turn off biometrics, lockdown mode, advanced data protection) but not default because they’re trying to balance being a luxury product with compliance, as a luxury product it’s used by rich people and rich people love their privacy (citation needed), Apple is the only practical choice if you don’t get a google pixel, FBI is always fighting them on allowing full phone encryption, they rolled out a contact encryption key verification feature for their own iMessage.

Signal explicitly doesn’t allow its files to be uploaded to iCloud. You practically will be fine using it on iOS. Unless you are in China which has its own iCloud/Apple servers, or the UK where Apple disabled advanced data protection.

Apple provides encryption for its customers, but not by default. All its encryption features hint to me that they would prefer not having anything to hand over to the hundreds of law enforcement agency’s around the planet, but don’t want to piss them off by making it default. This aligns with it being a hardware and service company, vs the advertising company Google.

Privacyguides has a page on steps you can take in your iPhone to harden it.

You are in a better situation than most. Make plans to get onto a graphene pixel. But if that wasn’t available then an iPhone is the next best choice.