In the last year Nurgle has only received improvements in points for units and battalions (which shows how much they were overpriced previously).
This has been joined by new deployment possibilities, such as the new S2D units (looking at you ugly marauders), or the wrath of the everchosen sub-factions.
Also, it was extremely important the faq that allowed the BKs to explode on unmodified 6 to hit.
Another thing that may have contributed to Nurgle's competitiveness is the shifting of the meta to the shooting phase:
this has worked in our favor thanks to the good average resilience of the units and has made even the PB a unit to be taken seriously in certain contexts thanks to the -2 to be hit and the 5+FNP.
That said, I do not believe that Nurgle is an alliance capable of producing 5-0 in tournaments, or winning them.
But with a good knowledge of the army and a very careful game i think it's possible to get 3-2 or, under certain circumstances, even 4-1.