Generally speaking, and let's say we're defining good vs bad as something which could do well at an average FNM.
With our current card pool, which 2-3 color combination is the worst?
I'd say its definitely Simic. U/G, as far as I'm concerned, has no real usable form. The only situation right now where I've seen it used is in the early days of Kaladesh when people were building Marvel decks as Simic, however nowadays its evolved into the superior Temur build.
I agree on UG, the combo lacks the solid answers that other colours have, and doesn't have the superior threats that we're used to from it. The counterspells aren't great either - having Revolutionary Rebuff and Negate as the only options isn't as bad as it could be but nonetheless leaves a lot to be desired when compared to other colour's answers.
With our current card pool, which 2-3 color combination is the worst?
I'd say its definitely Simic. U/G, as far as I'm concerned, has no real usable form. The only situation right now where I've seen it used is in the early days of Kaladesh when people were building Marvel decks as Simic, however nowadays its evolved into the superior Temur build.