I think you have to consider that these weapons aren't individually crafted by the developers. There will be a table with random enchants and when a weapons gains an enchantment it will be randomised from that table.

The result is that 90% of the time it won't often make much sense or synergise with the weapon in question but that's inherent to the nature of a loot driven ARPG. It kind of boils down to needing the majority of drops to be "meh" because then when something good drops its a genuine reward and not just expected. If everything that drops is good then gearing up becomes far too easy and weapon drops need to be even rarer so in most of cases these games now throw loads of randomised weapons at you and rely on you chasing the small % of them that actually are worth using. I'm currently playing Diablo3 and chasing "optimal gear" and trust me, I both love and hate random loot! xD

As for the punishment of 500hp, yes it's a fair bit for an assassin in particular :) but for a crusader who is very tanky, I think its viable but still not great given how rare elites are.. Obviously still only worth using on an anti elite weapon in the first place :D 

well take some reassurance that there will be 100 enchants in total, so the odds of getting it another 3 times in a row in final release of the game will be a lot smaller!

I do agree though that out of all of the perks, it's a little underwhelming. But.... Perhaps further down the line it might become more useful if say its you vs 5x dreads in some form of mission we don't know about xD

In the first place I am not sure if stats of a gun are displayed correctly at all. For example, if you get "specific weapon skill +damage%", It is most likely already included in stated skill damage. But if you get a general "ranged +damage%", or "skill group +damage%" like bonus to all ranged damage or bonus to Fleshbane skills, I am not sure if it is included in relevant skills. As such when you get "+Movement speed"&"-damage%"  you can't even evaluate well how it will affect a character.

Getting "-cooldown%" on skills that already have very short CD (0.6s or less) is pretty much pointless.

Getting "DOT +damage%" or "DOT +duration%" also grey zone as you can't gauge how valuable that DOT actually is - is DOT damage included in skill description?

I can confirm that when equipped that +skill damage is included into the calculation and works. 

To the best of my knowledge so is + ranged damage, but to clarify all you need is 2 weapons with the same base damage and differing ranged damage values to clarify. Fairly sure that it applies once equipped tho.. Because it's a character modifier, not a weapon mod.

Now dots... No idea because their dps isn't reflected in the weapons dps - comparing them is a lot trickier and always something a dps calculator will struggle to factor in.