Guild Wars 2

Sigil of Frailty should be way better than Sigil of Force in dungeons, right?
That is assuming you are not the only DPS (:D ), and you can keep up more than 1 single stack of vulnerability (keep up as in, having enough crit and staying at <25 stacks without the sigil).

Base duration 10 seconds, so effectively 13+ seconds. 6.5+ against bosses
ICD of two seconds.
Critting all the time with multi-hit aoe attacks.
=> 3+ stacks

So you lose 5% of personal dps. Assuming everyone in the group does even remotely similar damage, you’d get 5*~3% or more extra physical damage. Even taking into account how vulnerability stacks with itself (additively), it should be better.

Q: Did I miss or miscalculate something? Is Frailty being outclassed by another sigil?

It depends on your team comp and the particular encounter.

If you’re capable of maintaining 20+ stacks of vuln without that sigil, then it could easily be a waste. You’re also assuming that you’re fighting a single target, which is not always the case in PvE.

So it may be better depending on the situation, but not always. I’d say you’re always safer using a sigil of force, whereas the sigil of frailty would be used for meticulous min-maxing.