Is it relatively straight forward to complete all achievements on it?
Yeah, just get 100% in career which takes like 20 hours.
Single Player = good. Zombies - good. Multiplayer = fine. Game is glitchy though.
Zombies better or worse then 4
Better. Probably the only improvement to the game was zombies. The rest is...cod
Secret Achievement - Gears Tactics
Complete a mission without any of your units dying or being downed. - Gears Tactics
Kill 10 enemies. - Gears Tactics