Is Onslaught a joke?

[Shippuu]

It's a similar cost at 10 Kenki. Using both the out/in dashes will cost you 20, which you'll not only need to land a full 3 step combo, you'll also have to make sure you get the positional on the 3rd or you'll need a fourth! At 10 for the dash in, you'll need one or two GCD to recover from the loss, depending on what step you're on in your combo, you're landing a positional, or stabbing with Yuki. Both dashes also only deal 100 damage, but they save time on movement if you need to either dodge an AoE or quickly get to an add to burst down.

It is not a similar cost at 10 Kenki because SAM has many ways to generate Kenki, as every single move that uses the GCD generates Kenki, on top of being able to convert Sen into Kenki, and build it without a target in combat with Meditate. You talk about 5 Kenki = 60 potency, so a SAM's Gyoten is indeed at a DPS loss of only 20 potency, compared to the WAR equivalent, where 10 Gauge = 100 Potency, as Fell Cleave is 500 potency for 50 gauge, so you're losing 100 potency. On top of this, your dash out, dash in example is incredibly flawed as such. Using simply Yaten and then Gyoten is 20 Kenki for 200 potency, at a 40 potency loss, but you completely ignore the fact Yaten enhances their ranged attack Enpi, boosting it to 300 potency, on top of Enpi generating 10 Kenki. So for 1 GCD, when properly using Yaten, Enpi, Gyoten, you've only spent 10 Kenki, and done a combined 500 potency of damage.

You're trying to compare SAM and WAR, which is fine because SAM basically has the same resource system (on top of also having WAR's old stack resource system in a slightly different way). But SAM's version of it is far, far better than what WAR got stuck with in every single way, and has virtually zero abilities that are as wasteful as WAR's Onslaught.