The total disable time of her 'R' should only be about half a second or so - which is kind of a waste if you're using it to chain CC a team. It also doesn't do any damage - but the slow component is very heavy, I agree. However, it doesn't take the enemy team's ability to fight back (and fight they must now.) Put it another way, if you were to string 3 of Lulu's ultimate's together, you'd then have Sona's ultimate in CC length.
In my statement on Sona, I very carefully stated that the problem isn't AoE CC (Otherwise, Janna's ultimate would be problematic and I would've said so) it's about AoE "Lockdown" CC primarily designed or used to lock down the enemy team, preventing them from taking actions (or can be chained with each other). Howling Gale, for instance, is a huge AoE CC on Janna. However, actually using it as part of a Lockdown string is kind of a waste, as the interrupt value of it far exceeds the chain stunning value of it.