Is there much weight difference between the front and rear end on UTV's? Seems like most of the drive line is under the seat near the middle.
Depends on each individual make & model, also how accurate you want to get.
A single seat Ranger is 'fairly' well balanced not saying 50/50, most high performance blaster type buggies like an RZR are considerably rear heavy, not exact but closer to 30/70.
Same goes for ATV's, some front heavy up to 100lbs worth, some fairly balanced, some rear heavy.
Like you said " Usually load things by how they distribute the weight best on the trailer."
And that is the best most folks can or will do and it works out fine most of the time. When it doesn't...
Exceeding the weight capacity of the lowest rated part/component of your overall towing system is not 'legal' or safe.
I could list each piece & part but I think you get the point.
Most pickups & SUV's
without use of weight distributing hitch/hook up are limited first by the maximum tongue weight rating of the hitch mounted on the vehicle when towing with a receiver, ball & mount type.
Some use rating markings such as
TW - 500 TW = tongue weight
WD - 1000 WD = weight distributing
WC - 500. WC = weight carrying
WD - 1000
ST - 500 ST = standard
WD - 750
WDS - 1000 WDS = weight distribution & sway control
B - 300 B = bumper
R - 500 R = receiver
WD - 1000
Too much tongue weight doesn't always show up as rear end drop or sag.