While the Rotom forms do make useful scarfer/specs users, lack of a ghost typing really hampers their effectivety. Have fun with your sub+Pain split, Cune never really needed pain split and neither did Rotom foms (that is, before the loss of ghost type). Pressure is also better with RestTalk sets, and Rotom is just less useful in veery class it would try to use. There are far better status-spreaders (Suicune still can burn half the time WoW would, not that he's a status spreader) and better scarfers, and better specs users, AND better screeners. So if you REALLY need a Scizor counter, then you might want to use it. In any other case, it is just outclassedSuicune does not have access to Pain Split, Trick, Will-O-Wisp, Thunder Wave, Discharge, Thunderbolt, Volt Change, Electric STAB, or Levitate. Suicune does not have a Bug, Flying, 4x Steel resistance(s) or an Immunity to Ground attacks.
Suicune cannot preform Sub + Pain Split, Sub + Charge Beam, Choice Sets, Choice sets that use Trick, Status Sets that utilize Discharge and Will-O-Wisp, Dual Screens (Suicune does not have Light Screen), and cannot use Rest/Sleep Talk sets with it's new ability Water Absorb while Rotom can still utilize that without missing out on anything.
Suicune is a tank/bulky offensive Pokemon. Rotom is a supporter that can fufill both offensive and defensive roles thanks to his good Defenses and Special Attack.
They cannot be compared.