Why can some “always successful” moves be resisted and others can’t?

Dispels cannot be resisted. Everything else, unless it has the affix 'cannot be resisted', can.

Take everything I say with a grain of salt. I may be wrong.

Those debuffs that fail have a 15% chance of failure, disregarding potency and tenacity. Move's like R2's basic have the chance to try to apply the stun, and then the chance to stun. So an 80% chance to try to stun, and then it's got to get past the 15% chance of failing to stun.

DN and asajj don't apply debuffs, they just wipe buffs off. Because it's not a debuff, there is no chance of failure.

Someone else should probably confirm what I said, but that's how I think the game works.

On top of what everyone else said:

Tenacity Up and down cannot be resisted.

And I believe that answers every buff that can and cannot be resisted.

It’s bull I know.

The "check Sequence"

• Does it hit?
  
• will it proc (the stated % value in the description)
  
• Potency/tenacity check (chance to resist will never be less than 15%)
  

first one is pretty easy.
second one throws a lot of players for a loop, but simple enough to explain.
third one is a huge pain. but is bypassed by "cannot be resisted" in description. (tenacity down and dispell do not go through this check) ships have hard values so there is a lot less you can change here. increasing potency can help in most cases.

"Bennehftw;c-1314994" wrote:
On top of what everyone else said:

Tenacity Up and down cannot be resisted.

And I believe that answers every buff that can and cannot be resisted.

Really? So characters like bb8 will always apply tenacity down?

Yes, Tenacity Down is unresistable. (Unless the Attack was evaded)

DN not always remove. Dispel maybe can not be resisted, but this is dispel-on-hit (like Ezra) and therefore, it can be dodged - since the hit can be dodged. You see this a lot vs foresights.

After that correction, let's say - some skills say they can not be prevented, or resisted, or dodged. And that what gives them the upper hand on skills that doesn't state this specifically. Why some toons have and some doesn't? To make them stronger. Like han, as an attacker - he's amazing but not the best - and his utility of first turn give him advantage over others, and that's what the devs wanted to do.
If a toon is not strong enough, they can give him non-resistable skills to make him much better.

About luke not stunning - there's always a 15% chance to resist, after a tendown even. This chance only become 0 if the attack can not be resisted. (Han Solo shot first, thrawn fructure..)

Disclaimer about the unresistable stuff. If you are like Han (great example lol) but you try to stun zJyn, you won't be able to it. Zyn can not be stunned, so the Han stun is maybe not resistable, but just have no effect on jyn.

You can read more I'm swgoh.gg mechanics, very interesting. You'll see why Han/CLS TM removal is better than Jyn for GG in P1 HAAT, how r2 number crunch gains and gives the bonuses, and more and more.