Secret achievements are probably related to the main storyline which would explain why they are secret. If using the rest of the list as reference, I doubt they would be any more difficult.
Overall, this is a great list. The achievements are ALL very doable, require little to no grinding (except hitting level 50?), and just look like fun to get. I am tired of trying to get 1000G only to get bored shit of a game because I have to collect 82374985798345 flags or get 39475098745 kills. This is how achievements should be done. You can get almost all to all of them just by playing the game normally. You shouldn't have to go way out of the way and spend ridiculous amounts of time on them (I am looking at you Dead Rising with your 5 and 7 day survival achievements...).
I look forward to finally getting my first 1000G on this game.