Bit of history sperging to partially counter that point.
The Hessians (as they were collectively known since many of their commanders came from the Hesse area) were somewhere between soldiers of dual allegiance and mercenaries, as George III was of Hanoverian descent and thus ethnically German and thus many of the Hessians were working for German leaders who were in some way related or allied to George by marriage or political allegiance.
In practice, they weren't a roaring success, as they generally got curb stomped whenever they fought by themselves, and given what I mentioned above, they did have not only limits placed on them by their employers, they also could not embarrass their own leaders too much for political reasons, so they had somewhat more limits on their activities.
However, the British did make heavy use of Tories, Indians, and defector Patriots as mercenary forces, which often backfired (Burgoyne's Indian allies wound up being a lot more trouble then they were worth to him during the Saratoga campaign) and having fellow colonists turn on them only increased Patriot resistance.
In the long run, mercenaries didn't help them much, but by themselves weren't the only reason the British efforts in the American Revolution wound up imploding.
