So I've played Cassian and K2s0 twice and played against Iden 3 times.(only once against each other)
The marksman rule basically guarantees hits and sometimes crits. I've never seen the sniper rifles played because Cassian with his longshot and Iden's 5 dice cannon are both way better than almost every other gun in the game save a couple of lightsabers and tank guns.
Cassian has the K2S0 tax but i still contend that Iden is a little too good for her cost (first time I've thought that for this game, some things have been too expensive maybe but never too cheap)