Wont it be kinda difficult to find games, if you only play resistance/security??
Yes and no.
In Campaign mode you do indeed stick to one faction from the first to the last mission. However, the game will put you into a lobby and fill it up with bots, if there aren't enough players available for the same mission.
Ohter players can randomly be put in your lobby though, to play as your opposition (I guess they'll be picked from the Freeplay-Pool, then).