I think Jellicent's biggest issue is that it's typing overlaps with the two best spinners in the metagame, Starmie and Tentacruel, which makes it hard to fit into teams.
definitely this is the #1 problem. jelli is good on sand and rain (sableye is better than jelli when in sun). starmie is generally the best sand spinner, particularly the bulky kinds (donphan and forry are both meh, they both get destroyed by rain), and there is absolutely no contest for tentacruel being the best spinner in the game in rain. but if you run either of them and jelli at the same time, you become electric weak (cough thundurus-T)
also tspikes suck so subtox cruels (and in general, cruels carrying toxic) are becoming more common. they beat jelli one on one, of course. starmie will only beat you if you're really predictable and send in the jellicent straight away on the tbolt. sometimes you gotta mess around and play mindgames. LO thunder will hurt you regardless though.
nevertheless jellicent is still the only good defensive spinblocker available to OU (pain split sucks, don't run cofagrigus as a spinblocker, and don't EVER run dusclops) so already it's got a place. tends to also make a good genesect check - barring thunder, you can take anything genesect throws at you (you can take unboosted thunderbolts and recover them off), and moreover land the burn on that motherfucker, which is basically taking it out of the game. overall i would definitely say jelli is good shit right now; bulky sand in particular loves it