Roller is not really that easy to play. Hence why unprepared players keep getting killed over and over again with the roller. It is an easy to pick weapon, but extremely hard to master, IMO.
No, it's easy to use, just can be bested by someone with a ranged weapon and the ability to use it. I'm not counting strategems here as part of the difficulty, just how easy it is to work the weapon in a new-guy vs new-guy situation. Because we're in trouble if the game is only going to be enjoyable for the old dogs in the long run because they're crushing the newbs left and right.
The roller may be horrible at inking vertical slopes, but the fling attack has fair enough range (at least as good as Splat Jr.) and very good damage. The further out a target is, the less likely they are to be hit by multiple globules of ink (read: enough globules) and go down in a single fling. The roller is a pre-pump-to-fire shotgun with an auto-melee mechanic. It doesn't require quite the precision aiming a Splattershot does as it fires in a wave before the player, but in return, like the Splattershot Jr, it's more or less RNG to actually hit and splat another inkling, growing more favorable the closer the user is to the target.
The thing is, it will dominate low skill players because the options that clearly out range the roller require some actual accuracy from the player. The more accurate players can take a Splattershot or other med-long range weapon of choice and just mop the floor with the roller equipped Inkling, pun intended. For these reasons, it's best for a roller user to determine the skill level of their opponent, and choose to either run or press the attack. Generally, as a roller, it's safer to bolt down the very trail they just blazed, even possibly hide within it someway down and get their pursuer as they erase the trail. They kind of have to after all.
The best uses I've seen for the roller in are invading enemy territory and playing the hide and strike game. Gunning it straight towards an opponent that also isn't using a roller or Splattershot Jr. (aka, short ranged weaponry) is just asking to get splatted. Coincidentally, low experience players would do this.