My understanding is that Pix took over because the Brazilian gov did transfer payments during covid and the (only?) way to get those payments was via Pix. So it forced everyone to start using it. And once people had more familiarity with Pix, merchants started pushing for it because it charges ACH level fees.
The chargeback system (MED) is only so-so right now, but expected to get better.
There is a lot to like about Pix, but the spec is extremely complicated and hard to implement.
If you use an intermediary, implementation is usually very simple and straightforward. The biggest challenge is becoming a direct participant—that is, communicating directly with the central bank. In this case, you need to enter the regulatory framework and become a payment institution.
Many banks and Fintechs here offer this, the trick is that you need to open a local office, so you have a Brazilian subsidiary with a CNPJ number in order to open a bank account and receive the money.
Pix blew other kinds of bank transfer out of the water. TED, DOC, Boleto, were slow and expensive. Pix is instant, free for most people, and just works.
Pix and OpenFinance are extremely chatty APIs with a ton of rules. Being a merchant using the ecosystem is not that bad, but trying to be a participant in the network is complicated.
The chargeback system (MED) is only so-so right now, but expected to get better.
There is a lot to like about Pix, but the spec is extremely complicated and hard to implement.