Viper is not hard. It's painfully simple, in fact. I firmly believe the ONLY reason anyone is having trouble is because of the horrendous tooltips, complex combo system and their unbridled unwillingness to even try and learn it.
I'd like to preface this by saying I have Viper at level 100, and I used it in the recent extremes. That being said, I love Viper, it instantly became my main over Ninja, and hitting a dummy for even 5 minutes would give you a good understanding of how Viper functions. Unfortunately, some people cant manage that, and for some reason resort to depending exclusively on tooltips, which in Viper's case are terrible, or running to the forums to cry about how complicated Viper is.
Firstly, I feel like the lack if variety in the names of each skill are the first culprit. The words "Steel", "Fangs", "Hunter's", "Dread", "Maw", "Twin" and a few others are all used several times among all of Viper's weaponskills, sometimes upwards of 4 times. Of course if someone is looking at these weaponskills with nearly identical and often interchanged names will have trouble remembering which skill is used for which instance. This isn't even mentioning the fact that literally over half of Viper's weaponskills are UNASSIGNABLE, so unlike other classes, you wouldn't be able to hover over the skill to double check what it's purpose is, and even if you could, being shown "Can only be executed after Dreadwinder", you'd have to go back and look through all 4 abilities that have Dread in the name.
Secondly, the descriptions, the most important part, are vague as hell. Twinfang and Twinblood have "Changes to x, y and z when requirements for execution are met". Genuinely what the hell is that supposed to mean? What are the requirements? They seem to have zero issue dropping a ton of Viper buzzwords in the Coil and Den weapon skill tooltips yet cant manage telling us that Twinfang and Twinblood react to Dreadwinder, Pit Of Dread and Uncoiled Fury? This isnt even an isolated issue. Almost every unassignable weaponskill has the phrase "when requirements for execution are met." This is the cause of people complaining. A mistake on your part, SE, not how Viper plays.
Everything can easily be figured out with minimal practice. The positionals are quite literally a skill issue, not a big one either. Even with Viper's haste buff there is enough time to execute every necessary oGCD.
A better change for Viper would be making it harder. Simple things such as removing the highlights around most GCDs, removing the Noxious Gnash proc from Dreadwinder but making the GCD proc last longer. Give it a damn mit ability like Shade Shift, even a minor one. Games are allowed to have hard jobs, and high skill ceilings. For the love of the Great Serpent of Ronka don't make Viper easier than it already it.