This has nothing to do with imagination - it's taking a look at the numbers and realising the differences. Did your Marines simply not "get creative" as well? Do you regard a necessity to "get creative" in order to "keep up" fair if it is applied only to half the players? Don't you think that certain types of creativity might even go against a character concept? I could see an Assassin being very adaptable, but a Battle Sister notably less so.
But hey, if your players had no problem with it … lucky you. It seems to be the exception.