One disadvantage is that currently the UI framework is fragmented (UIQ vs S60) but this will be solved by the new Symbian Foundation initiative which will only support one UI framework.
Another disadvantage is that the OS is not available for PCs too. This has many implications, in areas like: development in native environment not possible, students are not learning to develop Symbian OS in the university since mobile software development is seen as a niche area, there are no easy to port applications from the PC world, there are no PC developers that can switch without effort from a PC project to a mobile one and back, or undertake an end-to-end PC to mobile project.
The huge advantage is the elegant design, created from the start for embedded systems, with limited resources and therefore with fine grained control over them. Teal time kernel, modular and extensible architecture. Sweet.
