A non exhaustive list of what makes them awful: https://expertbeacon.com/why-is-paypal-so-bad/

Afaik, the issue that was making the most victims, was that they were facilitating scammers that targeted sellers:
5. Good Luck Recouping Losses as a Seller in Disputes.
89% of sellers experienced dispute resolution problems with PayPal in 2021 surveys. Despite providing evidence the item shipped or service rendered, they lost cases and sums averaging $622.
This is driven by PayPal‘s buyer-favored review round taking 1-2 weeks. This favors scamming buyers at the expense of legitimate businesses.

Iirc, there was a time when Paypal always sided with the buyer, irregardless of evidence or past track record, the review process was useless. Once scammers picked up on this and began scamming sellers en masse, Paypal still kept their policy unchanged for years and sellers started to raise their prices on platforms that forced them to accept Paypal (ebay used to do this). Ebay has since tossed Paypal off their platform. I don’t know if Paypal ever improved.

The setup of this movie reminds me of Locke with Tom Hardy. That’s also basically a movie where you watch 1 person in a static situation (in a car in Locke) conducting remote project management. I’d have never expected a full movie to work with that setup, but Locke pulls it off imo.