I think the biggest problem w/ Mesmers in PvE is that you encounter enemies that use monster skills which are not as easily interrupted, don't trigger the various hexes you can apply, don't need to deal w/ energy pools, or just can't be stolen/disabled in the same way as normal skills/spells can be.
I like my mesmer, but it is often hard to be fast enough to interrupt the key spells/skills in large fights, and my damage is not going to be as consistent as an ele's
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