I ran a deploy to Heroku from my Ubuntu 20 PC.
It consistently failed reporting
npm ERR! fsevents not accessible from jest-haste-map
while loading the dependencies.
As I browsed StackOverflow, I saw a number of questions that mentioned fsevents in the pasted build logs, that's why I'm writing this here. It's a MacOs-only module.
The solution was to make fsevents optional with
npm i fsevents@latest -f --save-optional
I don't think this can be solved by this buildpack (can it?) but at least this issue and its solution could be mentioned in the troubleshooting section to save rookies like me a lot of blood and tears.

Apart from that, the buildpack worked fine. I could publish a client-only React app without having to learn about deploying, thanks you guys!