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I'm pretty sure the main problem with most all Simple users sans Swoobat, is that they have no reliable form of recovery. Also remember you have a limit of 4 moves which doesn't work with any of the Simple Pokemon because you need at least 1 attack and some for of recovery to make up for the damage done to you while setting up.
Not one simple user has access to boosting move for SpD and Def, an attack with atleast neutral to resisted damage on most to all types, as well as a recovery move. Here are two examples of what would be an ideal simple set up. *Indicates moves they cannot learn.
Bibarel
Ability: Simple
- Curse
- Amnesia
- Waterfall
- Slack Off+
Swoobat
Ability: Simple
- Calm Mind
- Cotton Guard*
- Air Slash
- Roost
Even so Bibarel would be stopped by burn and both it and Swoobat could be possibly crippled by any status move depending on the circumstance. A water absorber could also wall Bibarel. Numel could possibly work with an Eviolite and Stockpile, so long as it got access to an healing move.
I hope this explanation helped answer your inquiry.