It's the best out of 4 
items: 
(inefficient) 
(expensive) 
(scales up) 
.
It also gives mages 2 invaluable options: a) They can kill their targets in multiple rotations. They don't need ridiculous burst to be a threat. b) They don't need to hug the tower every time they're alone. The HP prevents them from getting burst down. The slow prevents them from getting rightclicked/chased down.
Sintama3 votesI hate how there's no reason NOT to build it. It works on every mage in every situation. You can even build it on
and watch Tibbers permanently slow and slaughter the enemy. They don't have a chance unless they fight Tibbers.
Dengeden1 votesYeah as I said, it's the best NLR item currently.
Still you can CC Tibbers. Or use cleanse. Or buy tenacity.
Talinis1 votesThe item needs big nerfs, as in, allowing it a 6 second or so CD PER target. None of this bs can't escape rumble flames, mord ulting you and you are perma slowed for the ENTIRE duration (i should not be slowed for 15 seconds from 1 ability) no more cass perma slow etc. Maybe if it was given the frozen mallet treatment where it had very low AP like 40 and then had 800 HP it would be better since they wouldn't have as much kill power if they get it early. But the item can't remain how it is right now.
Dengeden1 votesTrue. I was just pointing out what it did. It's annoying to be permaslowed but it's worse to be dead. If mages can't inflict you with the the former option they'll default to the latter again.
TequilaZombie1 votesWanted to upvote because of the 40 AP, 800 HP (which will make it a good item to it's intented users and crap to it's abusers).
Wanted to downvote because of the CD per target. It would make the item useless for the champions who rely on it and more focused on the burst mages who don't care about the 6 second CD because of their own CDs.
Talinis1 votesMages have PLENTY of options. Long range, build in CC, utility, good damage or great damage if immobile, ranged wave clear, siege potential, poke, zhonyas when they finally get caught. Most mages aren't mean't to 1 v 1, they are mean't to team fight, siege, wave clear and poke. Allowing them rylai's to endlessly kite and never get caught is broken and is going to be adjusted HARSHLY.
Dengeden1 votesI could list you multiple advantages for every class, just like I could list you ways around them.
I remember when there was that anti
circlejerk and if 
was brought up, people called it useless since you could simply apparently wait out the stasis.
And as you said, some mages already have CC, some of them will kite you "endlessly" or "never" get caught anyway. It's never been an issue up til now.
Talinis1 votesThere is a reason why LCS keeps picking Gnar with the frozen mallet, Cass, rumble and others who ABUSE rylais and not one single assassin who would normally COUNTER syndra/cass etc. And it's because they don't provide enough and rylai's just feeds into that. The item needs a nerf plain and simple and it's already going to get a nerf in some form whether to make it unattractive for most mages (having a CD per target, or go like frozen mallet, 30 or 40 AP and bump the HP). It's on Riot's high priority list and they've had many many discussions. Look to it being changed in preseason.
The mages who already can kite endlessly are those whose kits are designed for it and lose power in other ways to compensate. Just giving free endless kite ability via an item without giving anything up is not ok. Then you have lane bully cass and late game monster cass that just perma slows everyone and kills them because of the high AP it gives as well. You have mord that ults you and you are perma slowed for 15 seconds. Syndra spamming her balls. All these range mid laners that just keep perma slowing and that is NOT ok. Riot has already explained why perma slows are not ok and it is going to be removed. There is no reliance on it. The champions were fine without it before. They are just abusing it to make sure they get the kill instead of skillfully aiming their skill shots, making poor positioning decisions and getting away with it from rylais or in cass' case, no way to ever escape once she lands one spell.
Do you remember when they nerfed Nunu's ice ball slow duration, Janna's Zephyr???? The exact reason stated was slows shouldn't last an eternity and you shouldn't be perma slowed and be able to do something.
Dengeden1 votesThey don't pick assassins because the community got each and every assassin nerfed to the ground. There's now counterplay to anything and everything assassins do.
Gnar is picked rarely now that ranged tops except for Ekko are the norm but I suppose the point stands.
The point being that permaslows shouldn't be an inevitable part of any champion's kit. IBG, Rylai's and Mallet have existed for a long time, only they were situational items (and red buff has a limited duration and Nunu needs CDR)
(You can always CC or burst Cass...)
I'd just like to point out that you still need to hit one skillshot or get in range for a targeted ability before the permaslow starts. Furthermore slows into skillshots have always been a thing, particularly during teamfights.