The roller actually has a roller on the end of the lifter that follows the lobe of the cam shaft where the flat tappet's flat bottom does same. W/a roller you are able to get much more aggressive with the lobe profile (lift + duration) than you can on a standard hydraulic w/o issues. Much less friction as well.
To me it's win win but..........., in a mild motor you'd probably see nothing and they are way more expensive and more parts to break and go through the motor. ON a modded motor though they're all but mandatory plus a lot of OEM motors are now roller motors from the factory.