1. Everybody has Spring Attack. You can run in to melee, stab a guy, and then run BACK around the corner, out of range and next to the Defender. Even the wizard can "Spring Burning Hands" to line up a shot and then retreat to safety. Worse, if the PCs have formed a choke-point, enemies can use this tactic against them by having each monster run up, make an attack, then run away, clearing the square for a new monster.
2. Ranged attackers in melee can move back, shoot, then move UP to re-form the line (notably for our laser-cleric who picked up a shield and is now a decent front-liner). Taking disadvantage for making a ranged attack never happens unless you are truly pinned.