Unlike a spring washer, a nylon insert does achieve a very close contact with the bolt surface, closing the gap between threads closer than any other mechanical locking device. However, on-torque is required for this and, due to creeping, the gap develops over time, nevertheless.
Additionally, Nylon insert does not in any way contribute to the clamp load, which is the main requirement the threaded assemblies need to ensure (insert link to first article: Mechanics of a threaded fastener & factors that affect its reliability). In very simple words, it merely keeps the nut on the bolt.
Since quite a high force is required to efficiently tighten the nylon insert, galling can very easily occur during tightening, which also complicates the disassembly.
The same issue of expensive, large inventory of different nylon inserts’ sizes that needs to be kept remains, as it does for spring washers.
Liquid Threadlockers from Loctite actually address and resolve all of the mentioned issues, have a look: