Pros
Some nice people trying to move the needle. Good company overall goal. If you don't know anything about ML and want to be an ML engineer, they will hire just about anybody and give them a Sr ML Engineer title. Got a physics degree, you are a shoe in. Economics degree and one year at a "AI startup", you'll be leading ML projects and a Lead Software engineer.
Cons
Toxic middle management. No real experience in ML, but leading projects just because they've been at Square a long time (probably because no one else will hire them). Management is political, will promise new features or lie just to put up roadblocks against other more qualified teams that can deliver. The tech stack is an all inhouse nightmare, let's built it all ourselves mess. It's a bunch of engineers whose first answer to every question is , "let's build something new" or maybe if you are lucky they are like, "let's buy something new and then package a bunch of our own tech around it and call it a new project". Many of the projects are on the 4/5th iteration of just the same old tired nonsense so somebody new can be a DRI. Instead of, you know, just using open source tools and ML frameworks that ALREADY do what is needed. But there's no promotion or bonuses in making the right decisions in PIE. Maybe other parts of the company are better, but the PIE (ML Platform group) is career suicide.