It’s really easy to use. No buildpacks, no workarounds for Meteor-specific stuff, guaranteed to work with the next build (have had glitches when updating on Heroku & I think that mup sometimes has issues as well after new releases). Free one-click SSL. Barely have to even look at it or do anything. One-line deployment. Plus as a user of Meteor I want to support MDG in any way I can to ensure they keep development of Meteor. I don’t have the framework knowledge to really contribute to Meteor on the open source side, so I definitely support products they put out, even if they are slightly more expensive. You can have an app up in 5 minutes.
As soon as you start to need a full container or two for your app, the cost really isn’t too bad. One thing I would like to see improved is some sort of direct link between my Galaxy and Compose accounts, but that isn’t too big a deal. Also, a free staging server for people that have paid apps that works similar to Heroku’s free dyno tier would be nice. Other than that, I don’t know what more you could ask for from it.