The mercenary idea sounds very interesting, but as others have pointed it out, you would go broke if they are the core of your army.
The thing what makes mercenaries so interesting and unique is unlike most units, mercenaries become less valuable as they kill more and more. Usually, if you have a unit that kills things far beyond what costed it to make (lets say a single rhino tank somehow kills an nuwa cannon), you make a good trade as that is $900 vs $1800, not the mention that the rhino tank will get experience and become a veteran meaning increased combat potential.
However, the same thing does not apply to mercenaries. Although they are supposed to be extremely powerful for their cost, they cost additional money for each unit they kill. So lets say you have a mercenary tank that is the exact same strength as the rhino tank, but much cheaper (as it is a mercenary). Lets say that mercenary tank costs an initial $300 (being very generous), and you pay 20% of the cost of the unit that the tank kills. So if that mercenary tank kills a nuwa, the trade, instead of being $300 vs $1800, it becomes $560 vs $1800, as you need to pay an additional $360 because its a mercenary. If that same tank kills another nuwa, it becomes a trade of $920 vs $1800.
So unlike regular units, which give you better value the more enemies it kills, mercenary units are the opposite: the more units the mercenary kills, the less value you get.
With this in mind, I would definitely not use mercenary units for things like sneak attacks. Their good cost to effectiveness ratio would be hindered the more units they kill, so I would honestly think mercenaries would make excellent cannon fodder rather than actual combat units.
Also, we also needs to address what would happen if you are at 0 cash. What happens when you no longer can afford to support the mercenary?
