Pros
You will learn an incredible amount about the industry, working in tech and how to get better at your role. It's an extremely talent dense team - your coworkers are very smart, and will push you to be better (you will be expected to do the same for them).
It's a growing company and the work is genuinely interesting - no day looks the same, the team is invested in your individual growth, and you'll be exposed to most (if not all) parts of the business.
Leadership genuinely encourages taking time off - the expectation is to get your work done well, and take the time off that you need to do so.
It's fast paced - no red tape, changes are made quickly and transparently, and you don't have to wait weeks, months, etc to see and experience new processes/features/etc.
As other reviews have mentioned, there's no real hierarchical culture. You interact with and have access to all levels of people (brand new employees to the CEO). There's no ego - everyone is working with the same goal of making the software better, as quickly as possible.
Fulfil really cares about their customers - more so than any other SaaS company I've worked at. Customer experience and feedback are actually prioritized here.
Feedback is freely given and you're expected to give it as well - it creates a very transparent culture, and you will always know where you stand and what you need to work on, as well as what you're doing well.
Cons
If you don't identify as a self starter or can work independently with ambiguity, this is likely not a good fit. Due to the fast paced nature of the business, things are continually changing and adaptability is required.
There is a lot of work to be done - this is a rapidly growing company, and work-life balance can be difficult. You will need to navigate this for yourself.
The open feedback culture is a learning curve - giving and getting public feedback is not common at most companies I've worked at previously, and you need to become comfortable with this.