Its pretty decent because its weak to steel and thats kind of rare now without klingklang. Its only a 1 stage attacker. Many of them are 2 stage. It's both immune to status effects and can cause them. It 2HKOs pretty much everything aside from other grass types that resist water, unless you equip a bangle. Its a 1 prize attacker. Being non plasma non ex, it leaves a lot of other decks a handful of useless cards for them to draw into.
Either way, i just dont see VirGen being an extremely good deck in general. Its rather slow, and you burn through energy really quick. I seen and played a bunch of matches when the VirGen decks just flat out ran out of energy. Its even worse since energy retrieval is rather useless to this deck as well. The only way to keep going is to run multiple super rods, which then require you to use Virizion again. Sure this deck can OHKO anything. But so can Black Kyurem, and it can do it consecutively, thx to blastoise and superior energy retrieval. Though if it does run into safeguard pokemon it has jump through hoops to get around it. It just seems like Genesect is both its strength and weakness at the same time. Sure it can out right muscle through anything in the format. But it comes at such a huge cost.
Maybe people are just playing it the wrong way and should focus more on using megalo cannon, instead of just going for always for G booster as a crutch. Though then it becomes a mediocre attacker. It doesn't help the fact that deoxys doesn't really help this deck. Its a weird plasma deck that can't really make good use of other team plasma pokemon.