Why people dislike The Vertex so much ?

Vertex is fine - good rolls still sell for a bit too (although not enough to cover the mortal set sadly!)
The +1 has a lot of utility, chaos res is good for hybrid/ll. The 50% mana is basically wasted apart from if you wanted to stick arc in there I guess, or maybe hierophant might benefit.

it always makes me sad reading stuff like 'hybrid builds are bad' or 'only useful when there are no other options'

how easy nowadays opinions turn to facts and are repeated without checking. and opinions are formulated based on unsuccessful experiments

in short: hybrid IS THE WAY this game should be played. why it is played otherwise? mostly because copying builds is easier and it is also easier to min-max one stat than to min-max a multidimensional equation

some decide to bruteforce content with one dimensional build and extreme gear investment (like 8k hp 40k AR+ and nothing else) some try to make a hybrid with 2k hp and 1k ES, some believe that it is better to have 75% block and nothing else instead of having 50% block and 50% eva + 10k AR etc etc. maximizing one value is easy, seeing that survivability is NOT decided by one value is way harder

cannot imagine playing melee with something else than a hybrd: be it AR/EV life one or EV/ES/Life one, archer with some armor (yes, even with Acro penalty!) is just A LOT better than pure EV, spellcaster with some EV and AR (excluding flasks) is just sooo much better to play etc etc

in fact any PURE defence sucks in this game (ES pools work ONLY due to Vaal Pact - keystone that is a cancer to this game) and it is by design

as for the Vertex - a very nice helmet and pretty much the only option for EV/ES hybrids as people do not pick hybrid bases at all. the guaranteed Chaos roll offsets lack of life. but i agree that the rolls are all over the place with lack of focus

Archer with armour? Now I want to build some weird stuff again... rearguard block ranger xD

Vertex is a really nice helmet for housing Blasphemy-Enhance curses, or Empowered/Enlightened auras. The reduced mana cost part I haven't really found a good use for yet, but I still think the item is massively underrated especially considering how popular Heretic's Veil is.