First of all, there's no possible way to tell if one gametype is harder than another. People subjectively favor their favorite gametype or the gametype they're most familiar with.
That being said, Grifball is a hard gametype, just like MLG and Team Slayer, but for different reasons. Grifball doesn't necessarily require an inherent aiming ability (but it definitely helps, as evidenced by the fact that a lot of MLG and TS 50s play in the league), but neither does playing Team Slayer or playing MLG. If you've never played competitive Grifball, I wouldn't expect you to understand why Grifball is a hard gametype.
I think the easiest way to explain how 'hard' Grifball is, is to compare it to a gametype almost everyone should be familiar with - MLG. Playing Grifball is very similar to playing MLG. Like playing MLG, Grifball is a game of precision. It's not like Team Slayer where you can, for the most part, just spray your AR and get carried by your teammates. Like in MLG, a Grifballer has to make various different reads before the actual play begins, such as which launch to preform, sword vs hammer, adjusting for host and connection, knowing everyone's range and reload speed, and trick plays. The ranges, like a setup, constantly changes both before and during the round. It totally f.ucks with your game. You have to decipher (essentially just an educated guess based on past experience) how the range will affect your swing, which has a huge impact on when you hit your opponent. If it changes during mid-round, you may have to audible and change weapons or play a different style (defense, offense, 1-3 launch, etc.)
Playing Grifball competitively is completely different than playing in the XXP playlist. If you've never done it, you wouldn't understand. Similarly, playing on the main stage is hardly comparable to playing MLG in the playlist. If you haven't played competitive Grifball or played a s.hit load of games in XXP, you won't have any idea what's going on.
I've played MLG, Team Slayer, and Grifball competitively. My opinion > all of yours, except Pope's and anyone else that has experience playing a variety of different gametypes at a high level, including competitive Grifball. Also, Grifball is not my favorite gametype; MLG is. I'm not biased in favor of one or the other.
And to be good, Grifball requires more practice than any other gametype I've played.