Static-machine-per-AZ is trickier than one machine for everything (separate database/redis, need to transport containers around on rebuild, etc), but that’s just the cost of doing business in AWS, where there are no guarantees about the longevity of anything running in a single AZ. Autoscaling, on the other hand, requires automating a whoooooooole pile of stuff (like central container build, pulling down the container image from somewhere, ensuring the app.yml is available, dealing with in-progress rebuilds, and a bunch of other stuff), and then on top of that, automating it quickly enough that the autoscaling is actually useful (no point taking 20 minutes to do all that stuff because by then the load spike’s probably gone because everyone’s left because your site was down), so you’re probably going to have to automate the build of the entire AMI on rebuild, which is another layer of entertaining complexity. Don’t forget the fiddling required to gracefully transition between launch configs when you roll your new AMI – and somewhere in there you need to run database migrations…
AWS is not simple, and anyone who says differently is selling something. Probably AWS.