Morellonomicon...
This item is... imho... in a really unhealthy spot right now. So just a little preface for this. I am not a mid main, but as an ADC main, if I wanted an item with attack speed, crit, grievous wounds, lifesteal, and armor penetration. Well... it just doesn't exist, there's not even anything close. I mean, you can get a good mix of attack speed/crit or attack speed/lifesteal. And even with "meta" builds going around, an ADC still has build options to choose from based on their playstyle. A spammy Varus might want Essence reaver, a vayne that needs to kite a lot might choose botrk. Point is, by choosing those items each player is giving up something else. The essence reaver varus is trading a bit of basic attack damage he could get by starting botrk or IE for poke damage with the reaver. The Vayne is trading burst damage and mid game spike, for a safer duel item.
I don't want to make this post super long, but I could make the same argument with many other adcs, as well as tops, supports, jungles and even AD mids. But lets face it... when was the last time you saw an AP mid that used mana NOT getting morellos. And truly. why would they get anything else? This item is honestly too complete of an item. EVERY AP mage benefits from cooldown reduction, EVERY AP mage benefits from a large boost to AP, not to mention the build path gives them a ridiculous amount of mana regen for lane. Also, the grievous wounds transitions them nicely into teamfights allowing them to be a universal threat and giving utility. The kill/assist mana boost could not even exist on this item and it still would be great, but its like the icing on the cake with mid game snowball potential. Not to mention the incredibly affordable price right around what you would want for a first core item. Why would any mage NOT build this item?? I mean... that's great and all that mages have a good safe pick.
But it almost discourages creativity and the idea of "building for your matchup." I like how if as an ADC if you want to build grievous wounds, you give up last whisper. If you wanna build crit, you are likely bottlenecking into an IE at some point. If you want early lifesteal, you are sacrificing damage. It's a trade off. I feel like that doesn't exist for AP mana hungry mages. There is no trade off. Just get 900 gold and back for your first morello component like EVERY one else... It's just not healthy. I would like to see more diversity in mid lane.
Personal suggestion, put a little mana on banshee's veil build path, take the broken page out of morello build path, and honestly out of game would be nice. . If they want passive sustain they should be forced into a ROA or a Tear which sacrifices some of their damage. Make grievous wounds a conscious decision for mages to go out of their way to get, i dont even care if you raised it to below 40 or 50% health, there just needs to be some kind of trade off for it IDK guess this is just a rant, but I feel like the point is valid, and I'm sure I'm not the only one who sees this.
TL;DR- Morellonomicon is too versatile an item and takes away any mid build diversity. I feel like there aren't any real sacrifices made for the wealth of stats it provides.