What are the disadvantages of using THHN wire?

THHN's main disadvantage is that it cannot go anywhere without a raceway. Every foot needs conduit, tray, or tubing, which adds material and labor that cable assemblies avoid.

Second, pure THHN is dry-location only. Most modern wire is dual-rated THHN/THWN-2, which fixes that, but older spools and bargain stock may not be, and wet-location use then violates the listing.

The thin, slick nylon jacket that makes it pull so well also nicks on sharp bushings, and the damage hides until it faults.

For homes, that is why Romex wins inside walls and THHN wins inside pipe. It is not unsafe; it is just a conductor that depends entirely on the raceway around it.

Its high temperature rating is the upside; the 90C wire ampacity calculator shows when you can actually use it.