CP and N7 I honestly couldn't care less about. Ever since completing BotB I don't go for any specific challenges, they just sort of complete themselves. Mostly faction challenges, map mastery and the occasional weapon challenge. For promotions I have the amount needed for BotB with like 3 more or so more on top because I ran out of power reset cards. If anything I'd say some of the challenge grind could be tuned down a bit. Some of the 200 wave challenges were a serious kick in the dick to complete.
Manifest grind was pretty good though. It made me play with a lot of weapons and kits I otherwise wouldn't have tried and look forward to unlocking new ones.
I liked how DAI went about with their challenge system and the whole banner/title/portrait customization, that was pretty cool. DAIs RNG store was stupid though and had no real sense of progression. I had end game gear (well, end game at the time) by about 150 hours in, and once I had that, what was left? Without much weapon and build variety, DAIs end game is basically playing the same handful of characters and grinding promotions. Bleh.
I guess what I mean to say is that gameplay is what keeps me playing this game and any game really. Rewards are a nice bonus but aren't necessary.