We do have an actual real life example, for a large scale deployment.
Smaller ones around Africa.
It really does tend to come down to cost benefit, trying to leverage experience, and usually better equipment.
At the other end of the stick, you may end up holding it, and various inducements to get you to sign on, and an employer not too keen to pay out that success bonus.
If it gets attritional and long enough, equipment becomes cheaper, but maybe also reliable, since you know what works.
Everyone will try to get advanced combat rifles, assault rifles being a rather desperate measure, which did come up in a description of a mercenary outfit in GURPS.