They are, but some much less so than others. Even non-costume Lianna is a first rate sniper. And depending on your budget (or lack thereof) a second non-costumed Joon is often superior to your other options. Plus, there is always the chance of the costume down the road.
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I’m looking into a mirror here. 
I too have 3 maxed Joon/Lianna and 2 maxed Magni/Marj. Unfortunately, no costumes for any of them but I do have both Sartana/Quintus costumes.
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