Other than the online FOB stuff, you lose out on daily login rewards. Online FOB can accelerate base development, research, and resource gathering if you are active a lot. However, if you get targeted by an online griefer using cheats, you'll be constantly set back and have to spend the majority of your single player time just trying to keep everything at the same level, much less getting it to progress. Even without being targeted by a cheater, if you don't play the game for a few days or a week or two, expect your single player experience to get set back significantly as people steal your resources and your personnel every day.
Regards,
SB