I'm not much into the material, but how comes the F/SN servants are weaker?
To me, it seems pretty much the entire cast from Zero is stronger with the obvious exception that is Saber.
F/Z's Rider, Archer, Lancer and Berserker give me the impression to be on an entirely different level than their F/SN counterparts, F/Z's Assassin was pretty weak, yeah, but the F/SN one wasn't doing much more than have two major appearances and was little more than a roadblock to Saber. And that insane F/Z Caster was at least equal to the F/SN Caster.
Bluebeard is not close to Caster AT all. He is in the wrong container, the only reason he was even able to fight was because of his tome, once his tome is destroyed he is useless, this is not the case with Caster. She is from a divine period, her magic is on a whole new level to the point that even the strongest magus like Ciel could not defeat her unless she was able to get relics from the church to help.
Zeros Berserker is horrible. Berserker can only be damaged by A(I believe A+) attacks, Zero Berserker can only spawn weapons that are D, thus his NP is completely and utterly useless against Berserkers God Hand, he would be lucky to even claim one life.
As for Gil I will not comment on Gil and Archer because you will simply see that in April, but Gil is clearly weaker then Archer.
Cu fought all 6 of Stay Nights servants, survived and observed them. He was also able to stand up to Archer when they have a real fight. Also as Cu he also has his runes which can burn down a whole building. Zeros Lancer on the other hand had issues with Saber. So yeah.....
As for Rider, no way. First Rider has mythic eyes of petrifaction and has two types of bounded fields at her disposable which are EXTREMELY dangerous, her Bellerophon would just be another Ea incident if used against Ionioii Hetairoi.
As for Assassin, do note like Bluebeard, Kojirou is in the wrong container and is also a very abnormal servant as explained in UBW. True Assassin on the other hand is a real assassin. Both are stronger then Zeros assassin in combat(Kojirou) and stealth(True).