Talking about LB2, we are talking about several factors being changed over the previous Rolling Thunder Operation. The main troubles the US aviation faced during the whole war were, in my unexpert opinion, doctrine (training, weapon systems, planning) and politics concerning targets. Both of them were solved partially by the the time of LB2, but the US aviation was still unefficient in terms of air power for a given result. When compared with the contemporary conflicts in the Middle East, you can see how different an AIr Force performs with a doctrine adapted to its needs and the political will to survive. Probably, the IDFAF had more in common with the NVAF in terms of being Effed Up and pragmatical, facing an air force with internal problems, like the USAF or the Arab coalition
The reason it ultimately failed to prevent the South from falling a couple of years later was that, the way i see it, LB2 was not intended to secure SV as much as ending the war in terms satisfactory to the US. If Linebacker 2 was a failure, it is because none had the guts to launch Linebacker 3 in 1975.