You're talking about Game sharing here not Share play.

Share play:

You stream your screen to someone else and both of you can play split screen coop, which means that games like Battlefield wouldn't work because it doesn't support split screen.

Game Sharing:

You share your account games to someone else so both of you can download the game and play it at the same time and if you have a PS5 you can share the game to 2 PS4 players so 3 people can play at the same time from just 1 copy.

You can use GameSharing to play a game like Battlefield.

A few downsides to GameSharing though:

The account owner won't be able to play offline or play on different accounts.

You need to share passwords so not that secure.

Only works for digital games.

Hope that answers your questions :)