Pros
Flywheel offers an incredible culture. Specifically, you'll find that everyone you talk to at Flywheel has strong values and desire to contribute positively to Flywheel. Flywheel has quarterly all-company retreats where the entire company comes together (even remotes) to enjoy happy hours, workshops, and hear leadership talk about their vision for the next few months/years of Flywheel. The benefits are amazing and you'll find your job to be rewarding, no matter what it is.
Cons
As it's still a startup, often processes and opportunities you'd expect from established companies, are not always there. Career advancement, while available, isn't always communicated or made available across the entire company. Often, the folks who are most successful at Flywheel are those that are willing to work after hours (not OT), and pretty much bleed blue blood. While not necessarily a bad thing, it's important to recognize that a high level of commitment to Flywheel is required if you want to really move up or receive large raises. Again, not quite a con, but it isn't a pro because this sort of commitment could be perceived as poor discouragement of work/life balance.
If you work on the support team, expect to work occasional weekends (every 4 - 5 weeks) and up to two holidays per year. You can accept OT and double OT for holidays, but still, you'll be forced to work. I don't think you could really count those as a pro.