relax, we'll see Kenny's flashback of that fight when he's training with Unohana. For the moment, he lost because his opponent was stronger and smarter than him. End of story. He could have used the barrier thingy that made Yama used ZNT south... Royd knows all of Bach's technique + battle experience so it's not like he can't one shot a reckless shinigami.
So unless Bach beat Kenpachi, put Kenpachi down, then left, and then Royd decided to pick Kenpachi up again... I guess it must have been Royd who took Kenny out.
Since all the SR seemed to be captain tier, all Royd had to do was keep his Blut Vene active and punch Kenpachi unconscious. Not very hard considering Kenpachi usually takes a hit as long as it gives him a chance to hit back. But there are a thousand different ways Kenpachi could have been beaten, but it doesn't matter to list them all considering the Quincy are usually expert sharpshooters that fight with magic.
This is all speculation obviously. But Quilge definitely referenced Bach being able to adjust the strength of his soldiers' Blut.
So based on that, maybe the trade-off could be something like: Increase in blut strength means a decrease in switching speed?
Seems BV enhances defence and agility, whilst BA enhances offensive power.
Every spiritual being uses reiatsu when they attack. Concentrating the reiatsu flowing through their bodies is what allows them to make sword slashes that can slice buildings in half, and the like. Quincies are effectively human, meaning they should have human limits to their physical strength. Only by focusing reiatsu within their bodies can they hope to achieve similar or greater feats of strength than Shinigami. But that's only for melee attacks. It's probably safer to use a combination of BV and arrows like you said, but maybe BA + close range weapons inflicts greater damage.
So to take down Kenpachi, Royd would have used BV for defence, which we saw, and presumably gave him a good pummelling with BA.
"unless my blut vene is at full power, I won't be able to stop his slash speed but with my blut vene at full power, I can't switch over to my blut arterie & attacking without blut arterie is useless against someone is bankai mode!!"
You give & take away when it comes to Blut.
So, Unohana doesn't have to be on par with Bach in that sense.
He is being trained because he has immense raw power needed to be honed into some sort of skill. Without skill, he got owned by Royd and things won't change without a training, either.