The short verion of my opinion regarding memoirs by anybody: for the most part, the mere fact that the person in question was present at an event, whatever kind it may have been, seen something, spoke to someone, overheard something gives that person an edge over someone not there, etc. The only caveat, of course, is if it can be proved by documentary evidence that the individual was lying through his or her teeth. Always best to corroborate wht one can through other sources, and then use the memoir with whatever caution may be necessary.
Anf yes, I certainly know the caveats about Marbot and Laure Junot, but you can't beat good gossip with a stick, even when it is fueled in the first case by enhanced bravado and by drugs in the second.
I happen to think your version of Macdonald will be very good, regardless of the brouhaha over memoirs. More often than some folks would like to admit, women can see through the usual "perceived wisdom" about an individual like Macdonald, and come up with a much more nuanced picture. It worked for me, and it will serve you equally well.