Most GM games in the Marshall end in draws since it's been analyzed to death, so GMs are booked up on theory for many many moves. It's basically like both players using an engine until say move 30, and then playing on their own. Playing out a dead equal position on move 30 is likely to end in a draw, since there's not as much character left in the game. A normal chess game starts on move 0 and is also objectively equal, but there are higher chances to make mistakes due to the increased complexity.
To be fair, there are some lines where White can play a risk-free game even at the expert/amateur level. E.g., some variations have well-known possibilities for an early forced draw. Or maybe White can get a middlegame/endgame position that only he has chances to win. But I don't think the Marshall falls into this category. Black has excellent compensation for the pawn, and he is the one attacking. Of course, he must attack in order to prove compensation for the pawn, but I digress.
If you're worried about your opponent being booked up on theory, you could play lines like the Chigorin or Zaitsev, as these tend to be more closed and strategical. But if your opponent knows a lot of theory, then chances are they'll know how to get a slight advantage in these lines. Nowadays in the Ruy Lopez, Black's best systems are the Berlin, Marshall, Open variation, and perhaps the Arkhangelsk. I'd recommend learning the Marshall and one other system on this list, making it more difficult for opponents to prepare for you. If that's not a big concern then just playing the Marshall alone is enough.