I have tried to add this devotion to builds various times (including to builds without pets) but it always ends up inefficient compared to other setups also I have tried to think about it logically and came up with various reasons why I think this devotion is rarely useful and needs changes.
Soldiers, demolitionists and inquisitors are likely interested in physical/fire, but are all having better ways to apply damage reduction consistently on groups of enemies. Oathkeepers are often paired with one of those classes for phys/fire, so the usefulness of the DR seems limited to the oathkeeper+arcanist combo.
Suggestion: change the DR for stackable -OA, this is very useful for all the above classes.
For knockback its even worse, all of the above mentioned classes are already having ways to apply knockback with their phys/fire skills
The nodes of the devotion offer nothing special except +3 max aether/chaos, the rest is just very easy obtainable stuff, health, resistances and damage. +5 max resistances to pets is maybe useful in SR150? The devotion doesn’t really double down on tanking potential like obelisk and tree of life or ishtak (in case of pets). The devotions lacks focus, feels mediocre both defensively and offensively, pet builds like only taking the first node.
The devotion seems at its best made for a retal tank with pets doing the damage (since only then all stats would be used), but it is hard to think of a build that would actually use it as such since those builds would already have DR elsewhere and prefer other blue tier 3 devotions. Only combo I can think of is fire pet build with arcanist+shaman.