From a regulatory standpoint (the big hurdle in fintech), not so, which is why I lumped them all together. A single team/office could handle all the EU countries.
> presumably it's harder to roll out in poorer, less developed countries.
On the contrary, the more developed a country, the more regulation and red tape there is to deal with in finance.
Tons of developing countries have super aggressive financial/general regulations, for example India requires all data be stored in-country so you can't shard across geozones [https://www.lawfareblog.com/key-global-takeaways-indias-revi...]. In fact, the countries with the most complex financial compliance are almost all developing [https://www.bnamericas.com/en/news/five-of-10-most-complex-c...].
But the bigger thing is generally how advanced the existing finance infra is. Lots of countries have really slow bank transfers, processing a refund requires handing in physical paperwork, etc. Stripe relies on having underlying bank infra that's somewhat functional.
As far as the EU, the regulatory environment is manageble, but the payment landscape is very different from the US where Stripe originated. For example, in Germany only around 30% of users pay for stuff online via card [https://askwonder.com/research/german-market-what-s-breakdow...], that means for every country a ton of new payment methods have to be added for the service to be actually useful.