When comparing Magic of Incarnum vs Manual of the Planes (3rd Ed.), the Slant community recommends Manual of the Planes (3rd Ed.) for most people. In the question“What are the best D&D books from previous editions with ideas for Dungeons and Dragons 5th Ed.?” Manual of the Planes (3rd Ed.) is ranked 7th while Magic of Incarnum is ranked 10th.
Ranked in these QuestionsQuestion Ranking
Pros
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 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.