Without Grappler, grapplers use grapple + shove to prone. (Possibly prone → grapple with Shield Master). The enemy now has disadvantage, and melee users have advantage.
By applying restrained instead of prone, ranged attackers can attack with advantage (good). However, Grappler also applies restrained to the user, so the user's damage drops significantly – neutral on the target, disadvantage on everything else vs advantage on target, neutral on others.
In my experience, however, ranged attackers attack from as far away as possible (>100'), and so they aren't terribly vulnerable to the type of melee attackers that I grapple. (good not as good)
On the other hand, the grappler is crowd control – they aren't dealing a ton of damage anyway.
(bad not as bad)
Is there a way to use this feat that makes it better than Shield Master?