I really don't like league of slows.
especially when I play one the highest in combat speed champions.
serious 500 IGU/S is crazy.
that said, those slows make hitting skill shots more easy, a loot more.
so:
Rylai's:
Unique – Icy: Damaging abilities and pet attacks Slow icon slow affected units by 20% for 1 second.
Iceborn Gauntlet:
Unique – Spellblade: After using an ability, your next basic attack within 10 seconds deals (100% base AD) on-hit bonus damage (1.5 second cooldown).
Unique – Icy Zone: Triggering Spellblade also deals (100% base AD) physical damage to enemies surrounding the target and creates a icy zone for 2 seconds, which Slow icon slows enemies within by 30%. The zone has a radius of 180 (+ 55% bonus armor) units.
Frozen mallet:
Unique – Icy: Basic attacks (on-hit) apply a Slow icon 40% slow to your target for 1.5 seconds, refreshing with every hit.
Limitations
On Ranged role ranged champions, applies a Slow icon 30% slow instead.
the money efficiency:
frozen mallet:
94.09%
Rylai's:
93.83%
Iceborn gauntlet:
93.51%
but that is not the only "bad design" in terms of items.
also, people may hate frozen mallet more because you only need:
0,667 attack speed to apply permanslow.
while Iceborn need you to spam a ability every 2 seconds (the fact the slow is a zone can make it worse or better)
and rylai's need you to deal damage with a ability every 1 second.
and making a need passive will have people still taking the item, good point, but at least the aren't making you retreat or advance 20% to 40% slower.
(fun fact, Yasuo Q applies on hit, so he applies 40% slow with his tornado)
P.S.: i just think that Gnar is fun as he is, and he doesn't need frozen mallet to destroy people.
and things that make a pvp game more easy... you can say that is bad design.