Flip and Stealth are situational GPs. Flip works very well in Mayday and Mill decks for example, where finding the cards you need quickly is super important. Stealth is mostly for Hidden Rush deck strategies.

Thievery is the main Deception GP and is very problematic because of how powerful it is compared to GPs of other gods. It generates infinite value that is much more impactful than say generating 2/2 or 1/2 creatures forever. Thievery value is especially high against Magic and War opponents.

It doesn't grab spells from your opponent's deck, it shows you a random selection of 3 1-mana spells from your opponent's God domain.

Thievery is so problematic in fact that the game devs have acknowledged this and are working on alternatives. Same for Create. That's why I made this post https://www.reddit.com/r/GodsUnchained/comments/1fkc59r/ideas_for_replacing_thievery_and_create_god/ where you also have relevant dev quotes.

Here is another dev quote from today in Discord

And here is a small sample of the opinions other players have of Thievery: