Many gambits are unsound. The proof is told by the number of these played at the highest levels of the game. So, if you don't see the Blackmar-Diemer gambit at top tournaments, you can assume that its disadvantages don't outweigh its advantages. The King's Gambit is one famously busted opening.
That being said, we're not top players. Most of us can't beat the players who can't beat the top players. I'd wager, charitably, that 75% of the moves we make are wrong, technically.
In short, they're probably all playable. As always, if you're going to take chances, book up first. Understand the opening, know the main lines.