We have used Heroku a lot recently and it is so good.
At the moment you can't commit any script that does not support php engine prior to v 7.x.
That's good as the recent MODX releases do support php 7! Which is why you will never get anything like: remote: ! Push failed
See our demo: Heroku app based on modx cms for creating a custom invoice

However there are sometimes a kind of concerns about Heroku compatibility and the PHP engline. Well, trully it may have issues but working with PHP is still a good way. Here is a comment from youtube where some is pro NODE.JS and another one says that old (php) is gold ;-)

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