I own one and these are the best arguments against:

• much less upgradeable than a regular pc
• still possible to build something much more performant for the same/less money especially if you compromise on size (although this is less true every day)
• consoles still offer greater subsidized performance/dollar ratio

That said I love mine. I was nervous at first but even as a new release with some of the software kinks getting worked out it is still super low maintenance and has me playing more games than ever. I pulled the trigger based on my positive experience with the steam deck and I really do think it is a thoughtful, beautiful machine that is more than the sum of its (expensive) parts.