pros:

• faster compilation ?
• source code is more-or-less hidden

cons:

• limited debugging support
• can’t rename scripts or namespaces (all references will be lost)
• requires special steps to configure for building (e.g. to postprocess NetworkBehaviours)
• you need multiple DLLs - one for Editor and one for builds ; sometimes separate DLL for each platform ;
• need to configure preprocessor defines - need to copy them from Unity and configure for each build target
• not much open-source friendly (harder for other people to modify code and contribute)