I've seen numerous comments for, against, and undecided. Some have said it really only benefits Silver and Gold teams. Some have said it's just another $$$ grab by Glu.
MY opinion is if used correctly they are one of the best things to happen to this game. I am Platinum with a whole roster of capped legends and capped batting upgrades. Personally I only care about WOH and CVC. Do I need an X-factor to beat WOH? No, not really. BUT, using them speeds up my cycles exponentially. I clocked 1 cycle at just under 15 minutes this weekend. I manually play 16 games. 5.7 and the last 3 of each round. I only video boost on 5.7 and 5.10. Yes I even autoplay Tanaka. I had zero hiccups this weekend. Even managed 4 Bando repulls including on the 25th cycle. My best explanation of how great the X-factors are is using legend Adam Dunn as my example. He has the double hot hitter skills. Which means you start with zero active skills, which leaves you open to an unlucky match-up. Starting with a 6% boost gives you the best odds of nailing that 1 hit that will activate all 3 skills and then you cruise. On any skill(s) that turn off or on it will be the constant boost.
So you don't care about WOH? the Bonus Games X-factors benefit every team level. Platinum and below hit the wall at x-amount of games, the X-factors help keep you in those games. Diamond teams may not need the boost to win, but it will get them higher scoring games to increase club event points.
I have even used Prime X-factors to get more event points.
The only X-factors I personally think are useless are Season, H2H Match-up, and Tournaments. If you need help winning season games your team really sucks. I don't play H2H games. And the only level I see using Tournament boosts for is diamond.
You really want to fly through WOH use the X-factors.