When comparing Planescape: On Hallowed Ground vs Magic of Incarnum, the Slant community recommends Planescape: On Hallowed Ground for most people. In the question“What are the best D&D books from previous editions with ideas for Dungeons and Dragons 5th Ed.?” Planescape: On Hallowed Ground is ranked 1st while Magic of Incarnum is ranked 10th. The most important reason people chose Planescape: On Hallowed Ground is:
Several chapters are dedicated to exploring the potential interests, intrigues, machinations and mortal interactions of divine powers.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Divine interaction
Several chapters are dedicated to exploring the potential interests, intrigues, machinations and mortal interactions of divine powers.
Pro Real-world and D&D pantheon references
On Hallowed Ground details the gods of 20 pantheons, half of which are historical.
Pro Potentially better suited to 5th Ed. than v3.5
One core aspect of Incarnum is binding soulmelds to chakras, which prevents using a normal magic item in the same location. This caused scaling issues in v3.5, but 5th Edition already has a similar mechanic for magic item attunement.
Pro A truly distinctive conception of magic
In both flavor and execution, Incarnum is among the most unique concepts seen in D&D. Incarnum users form raw "soul energy" into temporary magic items grafted to their own bodies, providing a variety of benefits and abilities.
Cons
Con Missing some pantheons covered elsewhere
Notably, Deities & Demigods (1st Ed.) presents every historical pantheon in this book, plus American Indian, Arthurian, and Central American.
Con Extensive conversion needed
A 5th Edition DM wishing to introduce the concept of Incarnum must provide a full homebrew replacement for its rules. This is unlikely to change.