in the two GCD's assuming you're lucky to get it(this will change with 2pc as you're going to be more likely than not to get it) two FSc crits will do more damage than Obliterate/Rime even if both crit. Granted yes both are very unlikely to happen to get two FSc's to crit in a row without KM and same with Obliterate/Rime.
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Honestly, there's four ways to fix it. Buff Obliterate every few months, Nerf Frostscythe, or have Obliterate scale with more than just AP/weapon damage, or buff the shit out of Rime HB's even more so the loss of damage is worth the potential proc, because right now it isn't in KM situations. Nerfing FSc would be bad, because there goes our aoe, Buffing obliterate every few months seems like tedious work...and they don't seem to care that obliterate doesn't scale. So...here's hoping one of those things happens? Also, my FSc crits for 190-220k withot Pillar...My Obliterate crits for(both mh/oh) roughly 150-190k). So worst case..So yes..it sucks...but it is the only way to proc Rime so the final thing may be the best thing to do. However we could become TOO strong in aoe situations if they buff it too much,