But it's very much a niche card, one of the nichest. Which is both the problem and one of the reasons it's still better than The Card Which Must Not Be Named; thief is actively bad in 95% of games, but in the remaining 5% it's golden. TCWMNBN, on the other hand, is nothing in 95% of games, and slightly better than nothing in the remaining 5%.
I disagree. It's bad in more than half of games. It's usually not even worth the space it takes up. It's not as bad as Thief, but it's still usually worse than not having it. Most of the time it's worth it to trash it if you somehow get one.
I have never gotten Thief to work with Gardens in 2-player, and I have tried. It's honestly worse than silver most of the time, and silver is a pretty bad card with Gardens.