Just to add my pennies worth. This is not expert or inside info, but more ex-soldier and insight info.
My context is having served as a medic in a National Force. The reasons for the extreme difference of conditions between Mercenaries and a National Force, I would imagine, are the following:
BEHAVIOURAL:
They don't fall under the same military rules as a National Force would, and money is an essential commodity in the story. They will probably have a code of some sort, but thuis would not be the same as a National Force where you do something wrong and the sgt jumps all over you.
They will have smaller numbers and might not have the incredible infrastructure of a National Force as back up.
So they take much more risk.
Also, as a National Force, your safety is ensured even from each other, because the web of structure is so intense. If there is a row with another, you can fall back on somebody and appeal to them. If as a mercenary, if you fall foul of you boss, what do you do.
HUGE PAY
They are paid BIG money, because they don't have the same infrastructure behind them. Medical Aid, Equipment, Pension (If with so many risks you can look forward to enjoying that). Remember, as soldiers get older in a National Force, you probably start getting to desk jobs, but as a Mercenary, you need to make the cash to retire before you get to old (If you can).
Mercenaries need a cash attraction, because, unlike a National Force, will the helicopters come in to pull them out if it all goes wrong? You are without a government to pull you out, or an embassy to call (I may be wrong, but please correct me)
I have met soldiers who becamse mercenaries, reasons range from political (If you know the SA Government opinion on fighting in other countries - to serve, they have to leave the country and never return), through to money and of affirmative action in National Forces Re: promotion.
Also, bear in mind, a soldier of a National Force, who is full time, is paid wether there is a war or not. Mercenaries might need that money to tie them over between wars.
It has happened before that before mercenaries get to do their job, they get captured. Their families then need that money for maintaining the family, legal fees etc.
I would not want to be mercenary, even if the pay was good. And yes, they know their stuff, and I wouldn't mess with them.
I hope this gives a bit more light. I had just been thinking about these factors, and thought I should share them.
One thing to thank the Lord, is that your lads will have a helicopter that the Government has paid for, if they need to be casevaced, and leaders and men who will make a point of getting them back home.