Changing the Abyssal will harm melee and short ranged AP champs ( mid lane mostly )

As a Lissandra and Vlad player, I usually buy negatron cloak against a burstmage or AP assassin. Now that's no longer a viable thing to do, because the negatron cloak will be useless, as you don't want to buy the Abyssal anymore and the new Banshee's Veil buildpath is different ( you can only get 25 mr from the components instead of 40 like in the Abyssal's case )

You'll also lose a good chunk of your damage ( in lategame mostly ) because you'll no longer have the 10% bonus magic damage from abyssal.