Hours and work/life balance at SpaceX is likely the biggest and most-common complaint, but in my experience it is what each person wants to make of it. SpaceX will take as much as you want to give it, which for a lot of energetic fresh from college engineers is alot. There are lots of folks who have made a successful transition from 60+ hour workweeks to something more sustainable, but you must make a concious decision to do so.
For better or worse, there is definitely less structure, organization, and training at SpaceX than other companies. Nearly all of the training and learning is "on the job" training, as there are minimal mentoring programs. Also, some of the institutional protections of larger companies will not there at SpaceX (for better or worse).