Have there been proper apples to apples battery comparisons running same game at sameish resolution and same framerate limit? On paper the range of estimated battery life is quite similar 2-8 hours for steamdeck and 2-6.5 for old switch and 4.5-9 for new oled switch. The worst case 1.5 hours is mostly when intentionally setting excesively high graphic settings and uncapped framerate, on other hand large portion of switch games have simple graphics with framerate limit always enabled thus raising the typical battery estimate.

For simple 2d games both should have 6-8 hours of battery life. From what I could find for Witcher 3 (moderately complex 3d, but not the very latest) steamdeck can run 4H, for Nintendo Switch the battery estimate as claimed by CD project red is 2.5 hours, but some newer materials suggest that on the new oled switch depending on mode it can 4-5H. And at least initially witcher3 on switch in portable mode was running at 540p resolution. So again no clear winner without properly controlled tests.