In depth analysis of why Essence Reaver sucks and needs changes(has TLDR)
TLDR= Essence Reaver in its current state appeals very strongly to UTILITY based scaling adcs, and not mid game caster adcs. It needs changes to provide more immediate benefits early by itself as well to be designed more towards who it was intended on caster adcs. 30% cdr in one item is also taking up too much of the items power budget.
Essence Reaver was designed to be a first item crit option for caster adcs so they had an appealing option to scale with while allowing casters to sling spells in the mid game more often.
here's why it just straight up doesn't work and why very few people rush it unless they are going for pure utility focus (like ashe ER build or varus for example)
lets take a look at what ER does:
1.) 70 damage 2.) 20% crit 3.) mana passive (gain mana on critical strikes) 4.) grants 10% cdr up to 30% when combined with other crit items.
let's take a look at what IE does:
1.) 70 dmg 2.) 20% crit 3.) 50% increased crit damage (big dick crits baby)
both items suck by themselves, but ER somehow sucks even worse because you aren't critting often enough to really take advantage of its passive, and the cdr you gain extra from it doesn't even kick in until your 2nd item making it as a stand alone item even worse than IE which is already a terrible stand alone item.
regardless lets also stop and think about most caster adcs. who are they? who would you classify as a caster adc:

and you can make an argument for:

now lets look at who building reaver on doesn't feel 100% terrible:

weird? only one caster is in there? why is that? because the item appeals **UTILITY **adcs. not **CASTERS. **
but why does it appeal to these types of champs? look at the benefits of it again? its a crit based item which means it primarily benefits auto attack based champions first. so is there a passive that helps spells? on the surface yes! you get cdr/mana at two items. but if you think deeper you will see why it feels bad. here's my opinion:
1.) caster adcs like to be strong early/mid game. they fall off compared to pure auto based marksman because they lack the true steroids or range in their kits to (comparitively) keep up in dps. any adc is strong late game, but casters really want to shine early and mid. its why Lucian is only relevant when he has a build that makes him strong at 1/2 items respectively. if you force him into a late game build the only people who play him are people who just have fun with him! (like me, love me some lucian and regi)
2.) casters usually are already balanced around their mana issues. ezreal builds tear, Lucian has long been playable before Reaver existed. so why gimp damage when a good Lucian will just not spam his spells on cooldown knowing the right times to use them. everyone else just doesn't have mana issues so the mana passive is just a throw a way
3.) CDR is an excellent stat for casters, more spells = more damage. and combined with the mana one above in theory you'd think this would be great. casters could just sling spells all day during fights. the problem is though fights in league do not last 15-20 seconds. especially in the mid game where tanks aren't that tanky yet. people just fucking explode so you still only get at best two rotations.
so what does this make essence reaver? **A VERY GOOD LATE GAME caster item. ** But you know what you need by the time late game comes around? lifesteal, armor pen, and defensive stats in some form or another whether it be
or 
this leaves it little room in even a caster adcs build. but utility adcs? man they love this item. more spells means more utility for their team. sure they lack damage, but they hit like a wet noodle in the mid game anyway (except for jhin) and this is the best way to help them in the mean time. It gives them some damage amplifiers so they aren't 100% useless while emphasizing their STRENGTHS. (more utility spells) then it allows them to scale into the late mid game and late game with triple crit builds where they REALLY kick in line.
so what can you do to change Essence Reaver so it appeals as a starting option to CASTER adcs? Here are just a few of my ideas so take them with a grain of salt since I'm your average league of legends player and not some balance genius:
1.)
70 AD 20% crit 20% CDR
Unique Passive: Your spells deal increased damage scaling with your critical strike chance up to a cap (make your spells hit harder, but your autos don't get any added benefit)
NOTE: this would separate it from Infinity Edge and give casters what they WANT, more damage on their spells. Giving it a flat 20% cdr is enough. 30% cdr in one item is actually insane in my opinion and takes up way too much of the power budget.
2.)
70 AD 20% crit 10% cdr
Unique Passive: Auto attacks used immediately after a spell cast refund 30% of the mana spent on the previous spell. Unique Passive: gain up to 10% more cdr based on your critical strike chance (like live, but less cdr)
NOTE: this makes the item useful at ONE item for those who do want to spam spells. Without having to wait for more crit (since 20% crit you wont be critting often enough to get your mana back) you can immediately benefit from saving mana on your spells so long as you are slinging them out AND auto attacking. (think like sheen, but for mana) It could be a problem with bruisers, but since the item gives crit and you invest a lot of your gold into that I don't see this happening. especially since 20% crit by itself is just not that great. a bruiser would have to commit to TWO crit items which is terribad for them. Alternatively every spell can give your auto attacks a charge system (like riven passive) and you expend them within a certain time frame to gain mana back by auto attacking.
NOTE 2: this item would actually give Ezreal a viable crit item if for whatever reason the player wants to do that. the Q on hit auto attack would reward good ezreal players who land Qs by refunding 30% of its mana cost.
these are just some baseline ideas, there are probably better ways to fix the item, but right now it just straight up sucks. I'm a Lucian main and this patch I'll either still go bork or if I'm against a lot of tanks I'll probably go Infinity Edge. Sure I'll suffer cuz the mana nerfs, but even going oom faster than before its not worth it to build reaver because the item just gimps your damage and mid game too much.