Strong=!reliable.
Reliable means it's easy enough to land and not on a super long cooldown. Annie's passive is very reliable CC, as is Nasus's W or Ashe's passive, since they're all point and click (or can be), but not all reliable CC is point and click. Yuumi's Q is not reliable CC because you have to loop it around to actually slow someone - It's too short-range otherwise.
Meanwhile, strong CC is that which is highly effective. Morg's Q, Ashe's R, Blitz's Q....These are all strong CC abilities, arguably the three strongest in the entire game (All three turn teamfights on their own - Ashe's is the longest stun in the game, Morg's Q is a 3 fucking second root on a moderate cooldown, and Blitz's Q is death for any squishy). Yuumi's slow gets strong at a 80% slow at rank 6, but starts off at just a 40% 1.25s slow, and there are many people who call all slows except for Nasus's and Zilean's slows weak - Theirs are only strong (to these people) because it's a 99% slow from Zilean at rank 5 and Nasus's is up to 95% at rank 5 with a 47.5% cripple.
Calling Yuumi's Q slow strong is up for debate (it's definitely a strong slow once you hit rank 6 with it, but only a 40% slow rank 1), especially if you don't consider most slows strong at all, but calling it reliable is just plain wrong.
I am confused as to where the 60% came in, as well. It's literally never a 60% slow - rank 3 is 56% and rank 4 is 64%. No other ability of hers is a slow, and slows don't stack...Ghosts are a 40% slow and GLP is a 65% slow, and I can't think of any other slowing items she would buy.