Questions with x-factors and abilities

I'm returning to MUT after not playing since 17, so a bit confused with these and the best approach. 

Right now I have Malik Nabers, Hines Ward, and Earl Thomas that allowed for their AP to be activated. Why can these cards have their AP activated but not a card like 85 McCaffrey? 

It also appears you can activate 3 X-factors on offense, and 3 on defense. I'm thinking of activating these on my favourite players as they'll be on my team the longest, any suggestions on defence between Earl Thomas, LTD Bobby Wagner, Kam Chancellor, Richard Sherman? Are there certain x-factors that are considered the best/most useful? On offense I'll wait for the AKA Lynch to drop and probably not waste 100 training on another x-factor until I have a card that I think I'd use for longer. 

These are also costly with a cost of 100 training each... is the best way to get training to quicksell your BND cards you don't use? Any other good ways to get training, and what is eg. 100 training "worth"? 

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Another 3 unrelated questions so I don't make another thread: 

-How often are field passes updated? Eg. the Legends one gives a free 82 ... will it soon be updated with more challenges for a free 83 or is ea only nice at the start of the year to hook people? 

-Are there any packs that come into the store yearly that seem worth it? Eg. I didn't mind buying platinum packs during first week as you were basically guaranteed to get 100k per 1,000 points spent, now those already don't seem worth it anymore. I've heard redux packs mentioned here and there. 

-Is H2H regular season the best for casual/fun games? Champs is very sweaty I just want to have fun with the team I build, and looking at the rewards I'd probably go .500 at best and get 50k, could probably make that playing the market in a fraction of the time it takes to play 20 games, and instead play online for fun. 

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Thanks in advance for any input!

Cmc xfactors seems to be glitched and not working for alot of people. Training will be cheaper to get as the year goes along and card prices decrease. Quickselling BND cards that you dont use is a good way to get some free training and bidding on cards and trying to get them as cheap as possible is really the only way to get training or just quickselling a card for training instead of selling it. Ive always believed coins trump training every time and I try to stack coins and  just pay for training or qs cards I dont need or bnd cards.

Field passes have a timer that you can see and it tells you when it will expire for instance the season 1 pass expires in 2d4hrs and then season 2 pass will arrive. Ea is definetly not being nice with their legends and aka field passes cause those are terrible and only worth it if your already going to complete a legend players set. Though it is better than nothing but those two passes arent anything special. 

The best packs in my opinion is some of the store offers. Ive spent no more than $50 all on store offers and level offers and im up to 1.5m with an 82 Team. Im going to buy the level 35 offer of 2 grand elite platinum packs for 1600 points cause that's at least a good 180-300k+ at least. Rarely do I buy the regular packs but if you do then bundles are probably the way to go. They even have daily or weekly/promo offers that can be worth it.

H2H is sweaty also and can be very frustrating to alot of players. Make sure to know the stem route glitch and the best ways to try and slow it down or how to stop RPO's and stretch and other META that opponents will use against you over and over. I think House Rules is a great way to have fun online and to practice with your team. As far as the best xfactors /abilities this site has a list of all of them amd different tiers for them. It seems they've made alot of abilities as xfactors this year and they have players with built in abilities alot more this year. I believe each xfactors gives you a brief synopsis on how to activate it after you equip it. You do also get 90% of your training back if you decide to take away a players xfactors or ability.