What are some advantages and disadvantages for a Modular Kernel?

A modular kernel is an attempt to merge the good points of kernel-level drivers and third-party drivers
In a modular kernel, some part of the system core will be located in independent files called modules that can be added to the system at run time.

Advantages
The most obvious is that the kernel doesn't have to load everything at boot time; it can be expanded as needed. This can decrease boot time, as some drivers won't be loaded unless the hardware they run is used
The core kernel isn't as big
If you need a new module, you don't have to recompile.

Disadvantages
It may lose stability. If there is a module that does something bad, the kernel can crash, as modules should have full permissions.
..and therefore security is compromised. A module can do anything, so one could easily write an evil module to crash things. (Some OSs only allow modules to be loaded by the root user.)

http://wiki.osdev.org/Modular_Kernel