I mean running the same game at the same settings/resolution/etc the Deck will get significantly better battery life. You can offset this somewhat by using task manager to close background processes in windows, but it still takes a lot more power to run stuff on the ROG Ally than the deck.

An example from Ally reviews was Stardew valley ran at 6-8w on the deck, but was pulling 16-20w on the Ally with same settings. Using task manager to close windows processes for the Ally down to the 12-15w range, but that's still a huge jump. This isn't even taking into consideration that you can tweak deck performance settings to run Stardew valley at a much lower power setting, I've seen people go as low as 3.8w giving them 10+ hours of battery life.