Are legendary Pokemon viable without perfect IVs?

[Alfieri]

Since X and Y, legendary Pokemon have had at least 3 good IVs. Now in your opinion, do you think that their high stats are viable enough in an uber battle without 5-6 perfect IVs?

 

[Nah]

You can use them, provided those 3 IVs are in the right places. Zekrom very much wants Attack and Speed IVs, and if it doesn't have perfect Defense or Sp.Defense IVs it's not really gonna make a huge difference.

 

[Polar Spectrum]

Most legends are gifted with big fat HP stats, which can compensate for imperfect Def/Sp Def IV's. And their signature attacks with them crazy base powers usually more than compensate for any lacking Atk / Sp Atk IV. I think they're quite alright.

 

[Frozocrone]

The absolute most important IV for any Pokemon is Speed as it may mean you move first, but you can afford to drop a few IV's in some stats. I personally wouldn't settle for any stat below 20 - 25 though, unless there is no need for it (ie, 4 IV in attack for a Geomancy Xerneas)

 

[Megan]

With level cap or without? While having perfect IVs in all stats is important to max out a Pokemons potential on level 100, when playing with level cap (level 50 in cartridge games) there's a chance that your Pokemon hits a max amount in a stats without having perfect IVs in it. Although I can't say how much EVs factor into it; then again you'd want to recalculate them anyway, since Smogons EVs are optimized for level 100 fights.

If we're talking about level 100 fights, then it's important to know what your Pokemon does. If it's supposed to hit as hard and fast as possible, but dies to basically anything that hits it, you need perfect IVs in respective attack stat and speed while forfeiting some IVs in both defenses.