Very correct that the utility or passive abilities are very important. Some girls may have a better set of base stats than others, but a good set of abilities can totally sway the strength of a unit.
Iris for example, has "only" 4076 Attack Power when fully leveled and evolved and with maximum affection. Not quite as good as Rose, who has 4476 Attack Power when fully leveled and evolved and at maximum affection. But, Iris does have a 10% buff to Attack Power for all squadmembers, including herself. Effectively bringing her Attack Power to 4483.
Now, sadly Iris isn't a great unit overall, as most Pierce girls have a similar ATK buff and higher base Attack Power to boot, while Rose her greatest asset is her passive Counter ability, but this example should put things in persepective. It's about the combination of their statistics, their attack skill, and their passives.
Matching girls with compatible or similar passives also helps a lot with improving a squad overal. A single counter girl doesn't improve your squad by much. But 3-4 countergirls within the same squad, together with 1-2 girls improving their DEF or debuffing the enemy's ATK, results in a pretty powerful squad.
Once you get all your squads filled with gold girls, you can look at re-arranging them, to have them improve eachother. Even the least powerful girls can work out really well when you put them in a team designed to support them.
EDIT: link to the list of passives in the wiki here. It's really useful: http://harem-battle.club/wiki/Flower.../FKG:Abilities