I think hes viable enpugh that you will run some kind of ace counter in your list. I dare you not to.

The problem is that same things that counter soontir counter defenders so....you will run them anyway...

Depends.

If you want to counter Soontir Fel by taking zuckuss, crack shot homing missiles, and similar "dodge this!" attacks, then yes. Soontir, however, has a separate, arguably more easily exploited, weakness to stress - R3-A2 and similar are a nightmare to deal with for precisely this reason, and a lot of people (used to) take the 'cheap option' and put in stress mechanics to deal with aces.

Since TIE defenders are not massively susceptible to stress, this has become a lot less common as they become more common - which, if it's less prevalent, makes TIE interceptors more tempting again

Stress is still pretty common in the meta though, because it does screw with Defenders a bit (they're a lot easier to kill with only evade tokens, not focus/evade, and they can't clear stress whilst turning round unless they're Ryad who usually uses PTL anyway), and because it screws with a whole bunch of other meta bugbears, including but not limited to Mindlink (though not as much as some lists), Rey, Norra (and anyone using PTL generally), and so forth, not to mention enabling things like Latts crew and Slicer tools.

I mean, Assaj is great, stresshogs and stressmules are still excellent investments, and Rigged Chutes are still hilarious. Stress is back, because torpedoes no longer murder the stressors before they have a chance to do their thing much.