It looks like BB-8 Arc Welder applies Tenacity Down which is just a visual effect. I use BB-8 in the AAT raid and the resist rate does not change after applying the debuff.
When I use TIE pilot in the Pit raid the difference is noticable. Rancor with Tenacity Down resists debuffs much less frequently. But AAT resists at the same rate with or without Tenacity Down.
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It's worth noting that not all debuffs can be placed on raid bosses at all. They're immune to stun and shock for example, just by their nature of being raid bosses.
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/140717/im-a-very-lucky-person
But I hope Devs will explain the game mechanics in details some day. I'm not fond of any conspiracies, but it would be much a relief to be finally sure about my "luck" or possible "fluctuations" in the game mechanics.