Overview

Lootr chests appear normally, but "Graveyard" also adds non-standard containers in the form of coffins. I saw a similar problem with another mod and the creator of lootr replied that the compatibility issue was related to this other mod.

Why would this feature be useful?

Of course it will increase the possibilities of obtaining unique inventory for each player

There seems to be some issue with Lootr, again with Coffins specifically also.
LootrMinecraft/Lootr#412

Just to add a bit more info. Main issue is that Coffins count for some reason as double chests. Which breaks normal Lootr behavior, and force converting to Lootr chests splits them weirdly (but more compatible). But I don't recommend using Lootr for coffins specifically, so if you are smart, blacklist with Lootr config. If you aren't smart, either blacklist Graveyard from Lootr or don't grave-rob, Ruins are perfectly safe.