Infiltrators are broken because they do not fit the atmosphere of the game. They are not and cannot be taken seriously. Infiltrators are assassins, they are something people should fear.
No one fears infiltrators, they loathe them because they are annoying. They make your game experience annoying, dull, and tiresome.
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*log in*
"Oh, great I got stabbed again."
"oh goody, an RP from the "bad ass" infiltrator who never actually kills anything and is 'my worst nightmare.'"
"awesome, now I get to wait a week to play my character again."
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They just aren't fun in any way.