Some of the same things that are pros are cons. Allow me to explain.
Passionate employees - Everyone is motivated and trying hard which creates pressure to put in 100% every day, and at times work longer and harder when your team is at the plate to deliver. That said, the company avoids the typical "going gold" crunch by patching every 2 weeks (so there isn't a company-wide "crunch season"). There is also unlimited paid time off, a company retreat for everyone and wives/husbands/boyfriends etc (last year to Dominican Republic) and Rioters tend not to take themselves too seriously.
Incredible growth - Riot has growing pains in its org structure, support systems, IT, office space, etc. It's solving these fast - none of the problems last, and increasingly impressive people join the company to help build these areas out for the future. But Riot is growing quickly from a relatively small company to a mature company, and when some of the support has lagged behind it's been painful.
New Challenges - It seems you can never get too comfortable at Riot. There are always new challenges, evolution is the biggest constant. Processes continually get changed and improved, teams constantly have new talent joining, new projects are always getting spun up. For many this is the excitement of Riot, but you have to be willing to keep learning new skills and new ways of accomplishing your goals. If you're looking to sit back and relax, don't apply.
Autonomy - There aren't a lot of exact steps on how to do your job. Chances are nobody at the company has done exactly what they're asking you to do yet, so you need to make up your own best practices. This can be daunting and there may not be someone there to turn to with a playbook you can execute.
Cross functional teams - There are a lot of people with opinions and expertise in areas who don't think like you do. Figuring out how to get them all headed in the same direction can be tough. You need to be able to work together, be flexible and open minded. Also the bar to get into Riot is high - they are looking for great critical thinkers, gamers and aren't afraid to put you through a long and exhaustive interview process (and may pass despite great credentials if you don't meet their high bar).
Player-focused - Sometimes doing the right thing for players, and walking them through why you're doing it, is much harder. This mean that Riot takes longer on some features or content. Having to be in touch with what's going on in the forums, and players' minds is tough too.