Price + not worrying about scaling were the two driving factors.

In our Rails Fargate application, we get spike usage which is really annoying to manage if there's not enough "room". Sometimes Fargate containers will simply get killed if they're getting hammered to often and Fargate start time takes minutes.

Overall performance is almost on par with dedicated servers. One area it lacks is if Lambda decides to spin up a new instance, we get a coldstart time ~5s. It's not ideal for sure, but for our purposes worth the tradeoffs.