Mifune also hits twice. He also hits like a truck.

Both buff your lightning main, the difference is Dario only buffs lightning and water ninjas, Mifune buffs sword users,

So if, say, you had Wind Blade Asuma, Mifune is slightly better than Darui for a blitz damage team. However Darui has way more CC.

Ultimately you want both for a swords team, but Darui will also compliment your lightning teams

That guy is using Darui with that lineup because he doesn't have Mifune at a guess. I'm not saying he is wrong, or bad, but I suspect that is where he is going with that setup as he is using half-lightning buff, half sword user buff (and then iruka.... well, he is a stalwart). However, when/if he does get Mifune, the combination of WB Asuma, Darui, Mifune and MB is nuts