Has anyone compared pathfinder and 5e creatures (bestiary entries)? Are they at all inter-compatible?
Mechanically speaking, the likelihood of them being interchangable is extremely low as the two systems are already looking to be quite different.
From a fluff perspective, bringing / converting almost anything to the d20 system is not only doable but often pretty painless. However, if the monsters are at all similar to 4E monsters in their design paradigms, you will need to round out a lot o the monsters or else they will feel very boring and lackluster by comparison to other creatures in the bestiaries.
What I mean by this is, in the 4E design structure, the monsters exist only to be killed. As a result, they rarely if ever had anything that you could use to develop them as NPCs, or include from an ecological perspective.
For example, in D&D 3.x/PF, a bone devil has a few different things that it can contribute not only to combat but to the overall narrative. Their abilities like invisibility, greater teleport, and quickened invisibility make them great options for spies and assassins, while dimensional anchor and wall of ice make them useful for providing support for more powerful fiends.
In a similar vein, Imps have commune periodically as a SLA, which is purely a sort of story-forwarding divination ability.
You're almost guaranteed to stumble across undead fiends and creatures in a pit fiend's lair because of their create undead SLA which has little in-combat application. Likewise, their ability to trap and trade souls like currency (backed by their mechanics) in itself is a strong element that you could add to various adventures.
Further, most well designed 3.x monsters are more well rounded than they were in the 4E paradigm, often capable of changing their strategies up, with two or three different means of being an obstacle to the party (erinyes for example can provide martial offense, or blast the snot out of you with SLAs, in both cases usually as hit and run or skirmish tactics), which usually leads to more dynamic combat potential* than the default creatures in by 4E MM.
*: I say potential because not all GMs will run their NPCs to their potential. If a GM just has an erinyes stand around and shoot, or try to whack someone with their sword, and/or not take advantage of their abilities then the encounter will still be just as "meh" as it would have been if they only had 1 special ability.
However, a well designed monster is a character in its own right and when placed in the hands of a GM who will make use of its potential is leaps and bounds beyond what 4E-style monsters had in potential, though the 4E monsters were simpler.