Whatever type of team you put together, it is important to utilize their strengths. Because you can choose among the mercs, you can usually find one who meets the current need of the team. Be realistic about each merc’s stats, and increase what they are good at as they level up.
It’s good to raise some of their weak stats if they are in an acceptable range, but most are not jacks-of-all-trades. It is better to accept them for what they are slated to do and hire someone else as a specialist in another area.
Basically, a merc who starts with low stealth and agility should not be used like a ninja, and one with low health shouldn’t be used as a tank. This principle also applies to skills. Many of the mercs might be close to meeting some of the equipment requirements, but there is usually someone you can recruit who is already an expert.
More often it’s better to upgrade a merc to be more effective at shooting and surviving than pumping points into another area. Also keep in mind that barely meeting equipment’s requirements doesn’t mean that that they will be good at using that item. The overall determination of how to develop any soldier is what that squad needs and how effective they can be in that role.
Traits are another big factor in playing to the strengths of the mercs and the team. Many have proficiency with certain weapons and the 10- or 20-percent boost is something to keep in mind before switching them to another gun category or using them in an opposite manner.
Their traits are usually a good indication what they excel at and how they should be used on the team, but they are not the whole story. Always look at their actual skills and attributes before assuming that someone with a trait like explosives is really suited to that task. Some of the most telling traits are the ones like unfit, tough, or melee mastery that should give a strong indication where they should be in the battle plan.