Pros
If the tech stack interests you, you get to work with it.
Cons
Bad programming all around. It stems from the CEO, a bad programmer who grossly overestimates himself and feels qualified to make informed decisions after reading half a blog post about any given technology, all the while looking down at everyone else and badmouthing groups of people left, right and center. The backend is an unmaintainable mess, barely tested, done by ex-gaming industry coders (but not the ones that do the cool stuff) that simply didn't care one bit for readability, correctness, documentation, stability, or anything else. You'll be lucky if you are able to get any of their components working on your own environment. You couldn't pay me to run that crap on my production systems. Finally, the CEO is a terrible person personally, apart from being a bad professional. I know for a fact that ex Vivid Cortex employees have been shunned by Percona (Baron's former place of employment) due to them not wanting to have anything to do with him again.