The steps for an attack:
1 - does the attack hit (this is where evasion comes into play)
2 - does the ability produce the effect (if no % listed then it will always proc)
3 - potency/tenacity check (this is where cannot be resisted plays a role)
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Attacks that cant be evaded always pass 1, then you are subject to the % for step 2, and finally the potency/tenacity check.
The debuff isnt evaded, the attack is, debuffs by themselves can only be resisted.
When a non attack with a debuff is used and it states cant be evaded, then it will always pass step 1, then run through step 2 and 3. Yes it can be resisted or not produce the effect (if less than 100% is listed for the debuff).
