You don’t have to replace the entire logic board if your GPU explodes from too much l33t gaming. You can just swap out the GPU board for another one.
iFixit rates the repairability a 7/10, 10 being the easiest. As long as you can get your look beyond the ghastly sight of iMac components strewn about, it shouldn’t be too difficult. Historically, Apple hasn’t always made their devices easily repairable or upgradeable. It’s nice to see them taking a somewhat consumer friendly approach.