I talked about discordance practically a lot on my builds and cronos explains this quite a bit but I'll happily explain to you.
It is in fact a common but great mythic.
How it works. You probably already know this but it swaps mana cost, MH% DMG, CD and primary to secondary status and possibly APS depending on CD and/or weapon.
For example, if i have chakram and orb, orb will take on the cost, MH% DMG and CD of chakram. Chakram CD is 2 secs but cost is fairly low and gheH% value be swapped. Also swaps Multi Attack and Extra Attack from primary skill to work on any special skill.
That means orb gets 300% MH of the chakram . Yes this case be increased though for great DMG output .
Things that increase MH% when using Discordance Mythic: Push the Limit affix, Hero points, Identity set of different class and the fact that Multi Attack and Extra Attack works on them (usually extra and multi attack only applies to primary skills but this is discordance.
Usually orb goes great with discordance because of very high hit frequency and a huge increase in MH% but this case also be done for other builds.
An example could be Discordance Twisters, Storm, Aftermath meteors, Etc. Also discordance CAN affect procs so it goes great with Terrashaper .
A build using Discordance imo is worth it.
Here's some examples of builds that used this mythic:
Yeah pretty much all use orb but i did see a video using Discordance Twisters which are just as awesome!
Only discordance twister I've seen so far but it's awesome. Also some of those campaign discordance orb builds could have also used spam discordance Twisters for similar effect and damage but orb does great deal more DG and of course reactor set.