I dont feel that they are to OP because you can store them in ender chests as all this does is makes this loot not raidable if the player who owns it losses in a seige but this is the same as a chest with stone brick on top of it, using shulker boes is just alot more expensive way to store the items. So all this means is the only OP part of these is a player can store the equivalent of 13 and a half duble chests in one block but its minecraft space isent really a problem. The only thing i can think of that makes this slightly to over powered is that a builder can carry more material with them to a build but once again i dont really feel this is that over powered it has to be removed. There has been alot of changes in minecraft over the past few updates and I feel that if we where to start removeing anything that feels slightly over powered then half of the stuff that comes from the updates woud be lost, like look at elytra’s they let you fly in survival mod. So i think that if shulker boxes can go in ender chests it wont be the end of the world for raiding because all it is, is the same as haveing their chests blocked although this is only my opinion on this.