I’ve been watching the market for RRs for a year or two now, and occasionally I see guns that come with a blowback 9mm upper. The Egged-Hole Boogie Man makes me very hesitant to buy any of those guns, even when they sometimes seem like a great deal. Am I being overly cautious? Would that give you cause for concern?

I would want detailed pics of the pin holes on the receiver.
But that is good advice on any M16 RR.
Even then, I would only buy it if it was a very good deal.
Lots of M16/converted RRs out there to take a chance.

RCA

"Boogie Man" is an apt description.  With the early incorporation of ramped 9mm bolts, the root cause was quickly eliminated.  There are estimated to be around transferrable 10,000 RDIASs and 40,000 registered lowers.  How many actual receivers with egged out pin holes have been confirmed - not just rehash accounts of the same three or four over and over?

Perform your due diligence.
For a 40k weapon, it behooves you to check it out closely.
Egging out the pin holes may have happened on a very few registered receivers, but you don't want it to have happened to the one you buy.
Even repaired, that should factor into the price.
And anyone who has been around firearms for even a very short time knows that you can trust no-one.

RCA

I agree 100%.  My post above was not meant to imply that one should not do anything that you've noted.  And I would add, that as well as checking before purchase, one should perform the same diligent review after the firearm gets to the Class 3 Dealer if it is an out of state transfer.  Years ago I got burned (seller swapped out parts) and at that time, the state where the seller lived didn't permit non-state residents to file suits to redress such problems.

MHO, YMMV, etc.