Muscle Band isn't a bad item for competitive but it's outclassed
Muscle Band's boost is 10%;
>Effect: Boosts power of physical moves by 10%.
Muscle Band is outclassed in terms of it's boost really - Life Orb gives a 30% boost to BOTH Special and Physical attacks at the cost of 10% HP.
Expert Belt gives a 20% boost to BOTH Special and Physical attacks, provided they are super effective.
In all honesty, those two items are better than Muscle Band - I know of no Pokemon which would not work better with one of those items, or another one (Like Choice Band) compared to Muscle Band.