Compared to the first two serials, I think several of the performances throughout the third serial are inconsistent.  Don't know if there was something behind the scenes to account for this (shorter production?  smaller budget?), but I don't think this serial is as overall satisfying as the first two.  I still like it, but...I just can't put my finger on it.  (And maybe this one being more readily available for viewing compared to the others for so long might be a factor, as it was on several VHS labels before the other two.  I recall one local UHF station, after George Lucas commented the FLASH GORDON serials had inspired him to do STAR WARS, started running episodes of FLASH GORDON CONQUERS THE UNIVERSE paired with episodes of OUTER LIMITS as part of their late Saturday night line-up.)