The way you framed this question, the only merc units we can compare in a "presence of author/absence of author" way is those merc units who have had page time or novel time devoted to them.
That's actually a pretty short list. Wolf's Dragoons, Kell Hounds, Eridani Light Horse, GDL, Black Thorns, Avanti's Angels, Northwind Highlanders, I'm sure I'm forgetting one or three off the top of my head. Most merc units have only appeared in sourcebooks and field manuals.
I find this question kind of limiting. If they've never had an author behind them to provide that "plot armor" then they've always existed in the second state you mentioned. Always. They've lived there, their entire existence. Just getting a page in a field manual or a few references in a sourcebook. Sometimes winning, sometimes losing.
So I feel like you are asking, of the few merc units that have had that kind of author focus and attention, who do we think would still hold up and do well, in the absence of that author focus and just being treated like every other mercenary unit.
My answer to that would very much be it depends. As good as Wolf's Dragoons were they still got ground down on Misery and via employer betrayal essentially (yes they ultimately won on Misery, but the cost was enormous and it's not like the Combine recouped them for the losses). Any mercenary unit can fail just by being completely overwhelmed, 5-1 or 10-1 odds (the presence of hostile units that weren't supposed to be there), after having been given just all around bad intelligence and/or bad/poor/inadequate support going into a contract. That disastrous combination of many things went wrong, many mistakes were made, by the unit or the employer or both. Any of them can suffer a disaster from that.
So is your question then really, which units do you think don't deserve their success because of the plot armor they've been given, due to the dedicated focus of an author and page time devoted to them?