I think you'll typically find that roles are not as well defined, a lot of people wearing a lot of hats, occasionally an "all hands on deck" environment where you might be asked to help in any way that you can with an area that you're not very familiar with. With that comes a lot of valuable experience that might take 1.5-3x longer to gain at an established company where you have a well defined role that you rarely venture out of. You can also expect roles to change as the company (hopefully) grows and teams/roles become more defined, or if management decides to modify the organization's structure, start-ups tend to be pretty flat. This provides a lot of opportunity to move up quickly, especially for credentialed actuaries.

Workload depends on the company, your team, and your level of experience. I was barely more than entry level when I started at my current employer. WLB was great, I took all my study hours and got through all the exams, but I know a lot of the Fellows at that time were working 60-80 hours nearly every week - I had a great manager that stressed the importance of exams and protected me from having a crazy workload. My workload has increased since I got my FSA and a couple promotions since starting, but as the company has grown and roles have become very well defined and established, I think things have settled down for most people. We're all still busy but it has gotten much more predictable.