Luden's Echo is Cancer
I'm sure we've already noticed pretty much everyone and their mother is building 
This item is absolute cancer and here's why:
-
Luden's is here to replace DFG which was removed in the first place because it was used for gibbing squishies instead of busting tanks like it was intended to. Instead, Luden's is just DFG 2.0.
-
Burst damage is VERY marginally balanced - AKA most burst assassins have just barely enough damage in their kits to instantly take someone out in a fight ( <-- does NOT imply exclusively 100-0'ing them). To 'gib', a very strict set of conditions need to be met met (Level advantage, itemization, proper combo, summoners, landing spells). When you have Luden's Echo just dropping another front-loaded flat 100 base damage + 15% scaling AP onto a marginally balanced burst kit, those 'strict set of conditions' become very lenient. AKA imbalanced. -COUGH-
-
In the same way as Burst damage, instant waveclear is also marginally balanced. You need to meet relatively tough criteria to instant waveclear as a mage, Luden's Echo makes it much easier.
-
The concept of an AP
is dumb. Shiv is barely balanced because most AD Champions have weak waveclear and cannot even come close to 100-0'ing gibbing, not to mention Shiv is limited to crit/AS-scaling champions only who have even weaker waveclear and gibbing potential; But even then Shiv was too strong and needed to be nerfed - it was too cost efficient and gave a huge power boost to champions that were supposed to have a weak early game with its **flat **100 magic damage
. It was also problematic on ADCs that were already doing wayyy too much damage per AA 
. Now imagine if Shiv was a AD item (just like how Luden's is an AP item)
.....
-
On paper,
sounds balanced because 'hurr my gurd if I'm attacking Baron for like 5 minutes straight on annie only adds %x DPS"; However, in a real game, especially on mages, you literally only need items during your burst combo. Mages don't stand around dealing DPS, they throw their combo once and then go on CD. Having Luden's proc on your combo is like having far more valuable AP items right when you need them.
TL;DR: The concept of fks up balance across the board by distorting power strength and scaling. It gives you way more stats WHEN YOU NEED IT than most champions should have at X point in the game. It's effective gold efficiency is absurd because it gives you those bonus stats every time you pull your combo, which is basically akin to having way more items than you actually do.
Same as LB