I ultimately went with FlipBooklets (another deal on AppSumo) for what I needed and requested a refund after some initial testing. I recognize this is early in the development of the product and it will likely get better with time, but here is my review:
- Limited number of books for each tier (FlipBook allows unlimited books)
- FlipLink does not maintain the interactive fields in the PDF document when viewed in their flipbook, whereas FlipBooklets does
- Unlike FlipBooklets, FlipLink does not allow the user to add bookmarks as it only allows importing of predefined bookmarks when the PDF is originally created (which in my opinion is not useful)
- FlipLink allows for lead capture forms whereas FlipBooklets does not. However, there are some bugs with how FlipBooklets handles lead capture forms on desktop and mobile browsers which I reported to Sumit and he has indicated he will address them. Lead capture forms in a flipbook aren't something I needed so this wasn't a deal breaker for me.
- FlipLink allows for more flipbook backgrounds and other visual/audio customizations, though I didn't feel that it was enough to sway me.

Overall, FlipLink is great for what it does but it just didn't meet my needs.