So I picked up the Mutsu-no-Kami Yoshiyuki as I often played SAM, and had gear for a Seigan tank build but no Mythic (and no interest in doing so for SAM) and figured this would be a suitable placeholder.
It's not terrible.
In order to get it to really work to make SAM function as any sort of physical tank, you really need to go out of your way to find a use case, but it could function as a suitable 'Oh ***tank's dead' gearswap and hold hate for a bit while still contributing to dps.
Ideal case is similar to Ninja tank, you need a mob that's primarily physical with very little in the way of AoEs or magic that would wipe shadows. You also either need support, either a high buff situation where you can still reliably self-skillchain with defensive gear, or someone who can setup skillchains with you but won't out-DPS you enough to steal hate and make it a moot point. You also need a TP set that makes a lot of use of DA/TA since you do not have the benefit of a Hasso'd Zanshin.
Under those circumstances, I was able to keep the uptime of Third Eye up indefinitely, basically dropping the mob's auto-attack damage down to nil, while effectively doubling my white damage output with that weapon due to the counters and increased damage coming from the weapon and gear. Mpaca's makes a surprisingly decent tanking set due to the -PDT, M.Def and Killer effects the rare times Third-Eye was down, and with Third Eye up, it's safe enough to switch to a full TP set with just the Empy legs and get a really strong counter rate on top of TP. For funsies, I even tried it against 2-3 pulls at a time to see how it would fare and it managed well though Third Eye would strip in the last 10 seconds.
When it's not optimal though, things tend to fall apart pretty quickly.
SAM has no real hate tools, so there isn't any promise you are going to keep enough hate just by your damage alone (Counters generate no enmity). SAM has no natural MP pool so going /BLU isn't very viable, so you're either going /WAR or /DNC for a provoke (which means you lose dps by not going /DRG or /DRK). Holding hate is pretty vital because without the counters adding, your dps contribution drops considerably.
Second, without buffs, you simply don't have the TP gain available to consistently push out enough skillchains reliably -- Third Eye will strip and then you're in a holding pattern for 10-20 seconds until you get it back. There isn't simply a way to get ahead of it, and the 30 second cap means even a surplus won't last long.
I don't regret the choice, because the weapon has another unsung benefit, +40 ranged accuracy in the main hand puts Yoichinoyumi in the range of viable accuracy, without having to sacrifice STP for Racc, so I get to dust off my Yoichi SAM for the potentially 1-2 fights where ranged rps is the meta.