I think you're rather harsh on Manuel, there, and judging him with hindsight. If Manuel's a bad Emperor for leaving problems behind that lead to disaster, then so's Justinian, and Heraclius is certainly little more than an unmitigated disaster.
Manuel left problems that a better emperor (more like his father, say) wouldn't have left in the first place.
Justinian is not that different, and Heraclius is an example of a combination of circumstances, some of which we can't really ask him to have prepared for.
For instance (on Manuel), what was he (Manuel) doing leaving the Seljuks to do whatever in Anatolia for most of his reign? That was careless.
Manuel had too many projects and not enough lasting successes anywhere. At least Justinian can plead that his empire got struck by a plague. Manuel...just couldn't concentrate. He's like the anti-Basil (the Bulgarslayer). Not to say Basil's obsession with crushing Bulgaria was the best of all possible policies, but it at least accomplished that for the long term.
I'm treating the heir situation as beyond Manuel's control - its not his fault in any meaningful way that he couldn't sire a legitimate son until a dozen years before his death. Making a note of that because that lead to one of the really disastrous consequences ("No one after him was able to repair what needed to be repaired.").
As for the general Komnenid policies and style of rule, I think I agree with Sarantapechaina's main points. The Komnenoi were Anatolian dynasts, as you know, and, in the atmosphere of the 1070s and 1080s they prospered by basically taking over the Empire and running it like an Anatolian feudal estate, by sidelining bureaucrats, and establishing complex marital alliances. Komnenid Emperors were not really the "guardians of the people" that all earlier monarchs had posed as- they were, rather, very competent hijackers who used the Imperial system for their own ends.
I think one can go too far with calling that "feudal" however. It seems to be more a matter of...hm, how to put it.
Personal, as opposed to institutional, power. They drew on their supporters and their networks, rather than the bureaucracy every Byzantophile praises as one of the things that made the ERE a state and not a patchwork of princedoms.
This seems more true of Alexius than John (who seems to have had less trust in his family as a loyal base of supporters), Manuel I don't know.