Well for the callsigns unlocks I do feel accomplished. It keeps the momentum of the game as you have to complete a number of tiers to earn it especially for the scorestreaks as they do take alot of time to get.
As for weapon challenges, I have to say they are easily to unlock and personally I do feel that the camos should be hard to attain. I have already unlocked Diamond Camo on snipers and very close to completing the shotguns with just nearly 2days played. If it was to be unlocked on a higher prestige it would feel much more satisfying and considering camos dont reset when prestiging id say thats fair.
Just like in Black Ops 1 wer you had to be a higher prestige to unlock some of the stuff (Gold camo, colored clan tags..) but that was the only decent challenge in black ops loool.