its not the past 3-6 months, its been the past year of hard work of trying to make Nids "work" Like all armies were trying to do with the changes to 6th. It has been recent that people have been coming around to Nids being somewhat respectable which is a huge boost over where it was in the last year of 5th.
Nids simply gained more where other armies lost.
For example my favorite models in a nid army are their Warriors & Raveners - both units sucked HUGE MONSTER BALLS in 5th; with the high amount of meltas it meant that these units were literally being vaporized and they were effectively unable to deal with vehicles with A) not being able to hit reliably B) not doing anything from glances.
Now with 6th Meltas have been exchanged for Plasmas - meaning that my dudes survive longer...in theory
; vehicles are much easier to hit so my guys can glance them to death because of hull points.
that being said Nids' main focus (termagants & tervigons) did not change; it is the "auto includes" that have changed, no longer is it practically mandatory to take Hive Guard vs it is HIGHLY BENEFICIAL to take a flying tyrant.
Genestealers, which were a HUGE factor for the army in 5th, are now nonexistent.
what we gained was our respectability; what we lost was perhaps one of the greatest troop units of 5th.