Multiple currencies, trial periods, when in the process do you ask for the creditcard, country specific taxes,... And the worst: invoices.
While paddle.com takes care of many of those things - I was shocked after realizing how much work this is going to be.
Please think carefully about those things before you quit your job to work on your "Micro SaaS" or "unicorn startup".
I'd like to offer a free trial. While they have support for this, a user needs to enter his credit card right at sign up for trials to work.
So in the end I have to build a hybrid solution where my system gets tied to theirs. Still I don't have full control over all e-mails that are sent to the user. I'm not 100% happy, but better than not charging money.
For selling a SaaS worldwide, Stripe quickly becomes just as much of a nightmare. Taxes, billing, chargebacks, subscriptions, web hooks, different payment methods, VAT validation, card expiries, legislation, changing laws, SCA flows, and my favourite: SCA iframes that become non-responsive on browsers in a certain language. It is a never ending source of edge cases.
Don't ask me how I know... My biggest mistake with my business was that I vastly underestimated how much work it is to accept payments. In my case, it proved to be more work than building the product itself.