Funnily enough, I've never ever been able to understand what Heroku is, or how to deploy anything useful on it :) But I'm old, and I've always found it easier to tinker with nginx configs.
Tinkering with nginx is good, but what if you want to add a database, a cache, a logging service? What if you want to automatically build code when you push to github? What if you want to easy clone multiple environments (for staging, prod etc) without never doing an SSH into server? What if you want to have versioned releases that you can easily rollback to a specified version or push a branch into one environment and another branch into another environment? And you get a workable subdomain for every environment you build with SSL enabled. All of that can be scaled down and up with literally few clicks.
I... just add it. How do I add it in heroku where everything is "just create an app and push it" and everything is separate "dynos" (whatever that is), each with different pricing, storage, bandwidth etc.
Thing is: if we're looking for free, that may make sense. you still need tinkering though: you need to figure out what the whole dyno thing is about and how to connect from one dyno to another, and so on.
However, if we expand this a tiny bit to cheap and/or free, then simply running the lowest-tier server at Digital Ocean will be a better proposition. And installing a database these days is basically the same thing: run a command to install, all you need is to connect to it. No tinkering. And it will still probably scale significantly better than a free tier at Heroku :)
Once again, I'm biased and old, and can afford to spend money on side projects. I know that sometimes even $15 a month is out of reach (oh, I've been there). But yeah, this sorta prevents me from ever understanding Heroku :)