A thunder team right now would probably use both if you have them. It's not like there are currently any thunder Rs or SRs that'd be taken over them.

Tyr isn't really necessarily better than most SSRs. But Tyr isn't being compared against most SSRs. Tyr is being compared against other thunder kamihime. Thunder in its current state is utterly starved for debuffs, so Tyr's 10%/15% A-frame def debuff is quite valuable.
Also, the maximal damage against stunned enemies buff does have an insanely high upside to it, balanced against its narrow window of opportunity.