After experimenting with this addon, here are the glitches/bugs I found:

-Vehicle Spawning Issues:
Upon placing a modded car into the Messenger Pigeon, there is a 25% chance the game will crash immediately, and a 75% chance it will cause extreme lag until you remove the car. Vanilla vehicles work perfectly fine with no problems.

-Gravity Problems:
Almost all spawned entities and vehicles come out with zero gravity enabled, except vanilla vehicles which maintain normal gravity. This causes most modded vehicles and entities to float unnaturally.

-Map Compatibility Issues:
On maps with low skybox ceilings, the Messenger Pigeon either deletes itself shortly after hitting the roof or completely stops spawning anything, no props, vehicles, or other entities will spawn.

-Ragdoll Spawning Failure:
When trying to spawn ragdolls, the Messenger Pigeon immediately deletes itself and the ragdoll never appears.