Very easy to get started with, perfect for beginners, and seems immensely powerful beyond the basics (although I'm no expert).
None I can think of, frankly.
WebinarNinja helped me launch the first version of my storytelling course and secured my first sales - and it was super easy to set up and get started with. For that alone I'd be grateful, even if the product froze at that point - but Omar has spent the intervening years constantly improving it and working really hard to add new features that actually matter. I'm now planning to make recorded webinars a big part of my business in 2018 and WebinarNinja is how I'm doing it.