eh, I haven't actually had too much problem with Genesect (though I do run rain stall, alongside specially defensive Jirachi...). I mean, it's really good as a scarfer, but overall I think people are hyping it a bit too much.
Firstly, Genesect is so damn predictable. Basically, whenever you fight it, this is what you know:
It will have three out of these four moves:
-Bug Buzz
-Thunderbolt
-Ice Beam
-Flamethrower
It will have one of these two moves:
-U-turn
-Rock Polish
You can even simplify this equation further by simply saying that Genesect will always carry both Thunderbolt and Ice Beam, as those two moves are pretty necessary in terms of coverage. Now, I know that you could have all four attacks + life orb, or focus sash or something - the moveset doesn't have to be Rock Polish or Choice Scarf. But here's the thing: No matter what Genesect you're facing, they're all substantively similar. That is, all Genesects basically have 4 out of the above 6 attacks, pretty well always - and that means their counters are all the same. Blissey, Chansey, Heatran, Gastrodon, Volcarona, Specially Defensive Jirachi in Rain etc. These pokemon are basically universal Checks/ Counters to any Genesect. Sure, you could run that random HP [ground] for Heatran - but it'd be a gimmick, and you'd be walled by a whole lot more, thanks to giving up a far superior coverage move.
Now, don't get me wrong. Genesect is awesome. Fantastic. The bee's knees. It's just that, because everyone's been hyping it so much, I feel that I needed to weigh in on the other side of the equation, and take the role of Mr. Negative. Also, I keep on reading about Genesect's versatility - People. Genesect is many things, but versatile is not one of them. It's 2 major moveset options - Rock Polish and Choice Scarf - are stopped by the exact same things, and carry the same attacks bar one move (U-turn/ Rock Polish).
I feel like I've been too mean to Genesect, so I'll also say that Scarf (IMO it's best set) is amazing. With Download, even uninvested U-turn is solid, and it can potentially make it's Special Attacks scary. It's a fantastic 'mon, but like all pokemon, it has it's limitations.
Well, I suppose Arceus may be the exception to the 'all pokemon have limitations' thing, but you get the point :p