I think a BTN straddle is probably -EV. You are voluntarily contributing money blind. Without a BTN straddle you still have the choice as to whether to raise blind or not. BTN straddle is a restriction of your strategy, hence it must be -EV.

Now maybe BTN straddle changes game environment in some unquantifiable way (making the game more fun), and is therefore +EV for meta reasons. But from a strictly logical viewpoint it seems that it has to be -EV. Any limitation that restricts your ability to take actions will be -EV, no?

Notice that we are discussing whether a single BTN straddle is EV. This is separate question from whether agreeing to a round of straddles is +EV or not (that is raising the stakes of the game).

It really is going to depend on the details of the straddling rule. In some places I play, SB is first. In others, UTG is first, and the straddle acts in position order if there is a raise ahead of them.