"Original M16's were notoriously unreliable once fouled"
No, the powder specification was changed from the approved version without telling the designers so the appropriate modifications could be made.
Actually, the statement is correct. With the out-of-spec powder, the original M16's were unreliable. The issue was rectified in the M16A1, where pertinent changes were made, but it does not detract from the fact. Or the deaths it caused.
The other fact of note here is that the high-pressure direct impingement mechanism of the M16 is intrinsically more sensitive to clogging than the low-pressure long-stroke piston mechanism of the AKM or FNC, or the delayed blowback of e.g. the G3 and others.
Feed it with industrially manufactured powder, and it's no problem — but feed it with cartridges loaded with a powder prone to deposit slag and residue, and it'll be a very different matter. And in this setting, lacking factory powder, people may start concocting their own ... of dubious quality. And here's my point: powder that could keep e.g. the Swedish a Ak 4 or Ak 5 firing pretty much indefinitely, could clog up the M16 variants in the first firefight. Nothing that couldn't be cleaned of course, but it would render the weapon pretty inoperable until it was cleaned.