Its funny you should bring up the mixed scarf set.... because I was just going to add one the other day, but got distracted and stopped doing it.
The biggest issue with scarf is that you can not OHKO the weather starters. It's way too hard. In fact, here are some calcs...
Scarf+Naughty Lickilicky
252 Speed, 128 Atk, 128 SpAtk
Earthquake vs 252HP, 0Def, Neutral Tyranitar: 36%-43%
Powerwhip vs 252HP, 0Def, Neutral Tyranitar: 43%-51%
Brick Break vs 252HP, 0Def, Neutral Tyranitar: 54%-64%
Surf vs 252HP, 0SpDef, Neutral Tyranitar: 30%-35%
Powerwhip vs 0HP, 0Def, Hippowdon: 39%-46%
Surf vs 0HP, 0SpDef, Neutral Hippowdon: 37%-44%
Earthquake vs 252HP, 0Def, Ninetales: 57%-68%
Surf vs 252HP, 0SpDef, Neutral Ninetales: 35%-41%
Power Whip vs 252HP, 0Def, Politoed: 62%-74%
Thunderbolt vs 252HP, 0SpDef, Neutral Politoed: 31%-37%
Fire Blast vs 252HP, 0SpDef, Abomasnow: 91%-107%
As you can see, the only weather starter you're going to be (possibly) OHKOing with a Scarf is Abomasnow. Whats worse is that you pretty much fail to 2hko, even WITH stealth rock on some of them. And mind you, all of these have 252 HP, and are not invested in any defense at all, and all of them have neutral natures. Most likely you would be doing LESS than the above mentioned damage.
One last major con to a mix scarf is that you take away a lot from your bulk. A lot. Failing to OHKO a tyranitar means he is going to kill you. Politoed will toxic you OR deal 85%-95% back to you with hydropump.
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A special attacking scarf set with a 252 speed and a Modest nature would make Lickilicky faster than +speed nature +252 speed pokemon with base 80 speed. This gives you as much power as possible and lets you out speed all the starters (except ninetales) unless they are scarfed.
But even then, you're STILL only doing about 47% to Politoed with thunderbolt. 45% to Tyranitar with surf. 56% to Hippowdown with Surf.
Also, timid+scarf Lickilicky 252SpAtk, 252Speed... outspeeds Excadrill, yet fails to OHKO excadrill with surf. Would do about 52% (estimating). Which I am sure, an Excadrill will ohko back with a +2 Earthquake.