Cert manager is only for a subset of the aws offerings and I don't think it works for EC2 instances. I know it covers cloudfront, which means it covers static websites under https (CF fronted S3), Api Gateway (which is CF under the covers for the most part), so also Lambda apis. A seperate but same thing covers ELB. I'm not sure that it covers ALB yet.

That said for these services it works very well, except there are some issues with it lagging on sending confirmation emails that just make you retry.

It's been pretty seamless. We use it for our static website on S3 and for our elastic load balancer on elastic beanstalk. I'm guessing in a year all the other cloud providers will follow suit with similar offerings now that heroku and AWS both provide SSL certs out of the box.