as i understand physics and under circumstances you describe shorter barrel should have less muzzle velocity, less felt recoil, more dwell time less energetic (reliable) cycling, less distance on case ejection pattern BUT more muzzle blast and concussion as some of the propellant wasted on making big boom outside the barrel instead of pushing on the projectile inside the barrel.
from my experience i find that a lot of gun-owners are rather dim types who don't really have an education or understand physics and/or mechanics of a firearm on the level beyond 'where bullet goes out and where mag goes in'. Moreover many tend to parrot hearsay and some other ridiculous nonsense they've herd from some guy at the range. you won't believe what things people ask say at the shop often with having posture of authority and patronizing stance like they are Moses Browning or Mikhail Klashanikov re-incarnate.
short barrel rifle concepts are even less well understood and people tend to pile up different things from different sources regardless whether its correct or applicable in the instance. SBRs are far less common than non-NFA guns thus there is plenty misunderstanding and wrong info as the result.