The Morra is very decent. It's a dangerous gambit that has no refutation. I know of at least two expert level players that use it as their weapon against the Sicilian, not to mention Esserman.
While I can't speak to every serious black try against the Morra, I can say that Shankland's prep results in an endgame that is very slightly better for black (even material, black has bishop pair, development is nothing special for either side) with best play from white, so it's not like white is lost if black knows what they're doing.
Which brings us to the second part. The Morra melts people in practical play. Black has to be very careful to meet all the threats. Even when black gets objectively good positions, the game is still hazardous with a single instance of bad defense resulting in a lost position.
Is it a recommendation for a titled player trying to push for the next level? Of course not. Is it a practical weapon for club players? Yes.
As for the closed Sicilian, I considered including it. Probably should have.
@23
"there are three anti-Sicilians that are decent and somewhat simple: Alapin, Smith Morra, and Grand Prix Attack." ++ Closed Sicilian is decent and simple: played by Smyslov & Spassky. Morra is not decent: you lose a pawn for some insufficient compensation. Moreover, black can decline the gambit and transpose to the Alapin, so the Alapin is better in the first place.
"Grand Prix Attack (2.Nc3 Nc6/d6/e6 3. f4): Cavemen chess"
++ There are 2 ways to play the Grand Prix: caveman style with 4 Bc4, or positionally with 4 Bb5, like a reversed Nimzovich Indian Defence.
"there's no simple ways to attack the Sicilian" ++ That is right: Sicilian is sound.