[Klashikari]

There are a lot of misconception, although something has to be said: comparing servants in this fashion is redundant, since Nasu has the knack of using way too much convenient/circumstancial situations to dictate a servant strengths/weaknesses, even though there are things completely ludicrous (such like SN Lancer's gae Bolg and SN Assassin's Zabaniya)

First and foremost, Dark Berserker cannot be a good example of factoring Excalibur strength. The very reason is two fold:
1) There was no description of how many lives Excalibur could take out from Dark Berserker and likewise, no description how many times he died due to Nine Lives Blade Works (the fact it deals 8 strikes doesn't mean it actually killed him 8 times. Likewise, the finisher attack was a NORMAL thrust, it wasn't part of NLBW).
2) There is no indication if God Hand was still active and/or if Dark Sakura's prana didn't replenish Dark Berserker lives stock (since she has unlimited ressource of Prana as the holy grail).

Consequently, concluding that Excalibur "full force" only shaved 2-3 lives is really too rigid in term of theorycrafting, especially considering how Caliburn was vastly inferior to Excalibur here. Dismissing this point as "plot point" makes the whole point moot since you are conveniently cherry picking one instance as "mistake" but the rest as canon, which is doubtful at best.

Now, there is also another potential explanation for that: Dark Sakura did not want to kill servants, because she doesn't want to lose herself. This was also commented by Zouken, stating that she could kill Saber and Berserker any time, but chosed not to for her own agenda, but would make her disperse way too much.
With that in mind, she probably ordered Saber to simply defeat Berserker, so her shadow would be able to swallow him completely.

Another point that must be taken in consideration is SN Berserker: God Hand only provide protection against B ranks attacks or lower, but that does not only stick to Noble Phantasms: if physical or magecraft attacks is by itself a A rank, it -will- damage SN Berserker (as proved numerous times, such like Rin's attempt in Fate route, which actually killed him once... don't tell me that Excalibur in full force would barely deal twice/thrice than Rin's desperate attempt).

God Hand -is- an extremely potent Noble Phantasm, but it is by no means impossible to overcome alone. Should a servant have A rank stats + A rank NP, they can potentially kill Berserker alone, provided they aren't killed first.
Which means: with several A rank attack due to stats and a NP that can shave several lives at ones, it is doable alone.

I personally see SN Berserker and Zero Berserker as equal matched, although they both have favored situations due to their respective NP and fighting style. Both are beefs and would be much more threatening if they werne't Berserker class. Really, basing on their stats alone is ludicrous at best, especially when we are dealing with Nasu and his usual shenanigans.

By the way, remember that unless someone has a high degree of Divinity, Enkiduu has as much strength as a regular chain against a Heroic Spirit without any divinity attribute.