I'm not so sure the POI shift with a suppressor vs. without is primarily or solely due to the weight of the suppressor. Sometimes just a muzzle device can affect POI in some cases. In some cases just the miniscule effect on the bullet exiting the barrel by any means will affect POI. Don't get me wrong...I'm not saying putting weight on the end of a barrel doesn't affect POI...it does. Even a short, stubby, beefy, pistol barrel's POI can exhibit POI shift with a suppressor, and it probably has more to do with the pressure curve and other "black magic" physics issues that can affect a bullet as it leaves the rifling at the muzzle crown. It's not a simple "one issue" deal IMO. No doubt that barrel harmonics is a big issue due to weight and contact with the barrel. It's just not the only one for sure.
On barrel nut integrity under fire, there may be some AR10-style platforms out there that have wimpy barrel, barrel nut, and barrel nut extension issues, but I'm not aware of them...and if there are they are probably not on most people's radar as a good .308 rifle candidate. As to the pencil barrel/heat issue, I think it takes some fairly decent rapid fire conditions to make this a big issue...and under conditions where you're pumping out rounds that fast, is accuracy the all consuming goal at that point. Maybe a belt-fed .308 would be more appropriate.
In a semiauto, AR10-style weapon, I wouldn't go over 20" in a barrel. When long range accuracy and effectiveness become that important, a precision bolt rifle like the Ruger and others will probably be a better choice. This newer generation of chassis, precision, bolt rifles are pretty cool and offer quick followup shots as they're magazine fed and have short rotation bolts, AR furniture options, etc. I have a homebuilt chassis bolt rifle and an Armalite 20" barrel .308 AR10. The AR10 is a fun and decently accurate rifle, and it would be my choice for a stationary location, defensive weapon like a "Firebase Gloria" scenario...a Vietnam, trench warfare style of situation where big, longer range, semiauto/full auto firepower might actually be a benefit. My two AR10's were my first AR platform guns...a 16" and a 20". Now that I have a bunch of AR15's, bullpups, and a precision chassis bolt rifle, they're probably the lowest on frequency of use or choice in SHTF scenarios.