Are mixed sets viable in competitive play?

Question

Just in general no specific Pokemon.

Comments

This hugely depends on what Pokémon you're referring to. I'm pretty sure that without a certain Pokémon, this would break 1.4.1. (And possibly even 1.4.2?)

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I think the answer below shows it's possible to speak broadly on this topic, but I get what you mean.

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What format/rules are you asking about?

Answer 1

Yes, mixed sets are viable in most formats.
Pros
1. You can hit both physical and special walls on their weaker defensive stat.
2. You often have better type coverage overall.
Cons
1. You're often forced to run a Defense, Sp.Def or in some cases a Speed dropping Nature to get the most out of your attacking stats.
2. Sometimes you have to sacrifice speed EVs to hit a benchmark on one of your attacking stats, or vice versa.

An example of a good mixed attacker in OU is Arctozolt. It can 2hko the physical wall Buzzwole with Blizzard and 2hko the special wall Blissey with Bolt Beak.

Comments

In particular, mixed sets are generally less powerful than other sets because you can't/shouldn't put 252+ on both attacking stats.
I would emphasise that mixed sets are niche, and most Pokemon don't have a real reason to bother with them. I would also make a distinction between a "genuine" mixed set and something like HP Ice Landorus-T in Gen 7 OU, which had such low commitment that you'd run Jolly on it.

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Its useful but it makes Evs complicated