I'm not sure if limiting the factions would be helpful (almost every faction has Ambush, for example) or in the spirit of OPR mission design. Since Ambush is probably the biggest issue, perhaps a fix might be to add a rule along the lines of, units that aren't on the table by the end of like round 2 or so don't count towards the total? Or alternatively, adding some kind of additional objective that the Seekers need units on the table to achieve, such as finding and/or securing an exit.