I don't know what you have against Hashirama/Tobirama, but I wouldn't call either of them weak. Hashirama and Madara are regarded as legends amongst modern shinobi. Hashirama is the more powerful of the two (since he beat the other in combat), and even Madara seems to regard his former opponent as exceptionally powerful. Tobirama isn't exactly a slouch either; afterall he created the Impure World Resurrection Technique. Furthermore, Orochimaru's version of the technique seems to be fundamentally less powerful than Kabuto's. This really doesn't detract from the people being brought back so much as it indicates that Kabuto's use of IWR is just more advanced.