Generally speaking, the smaller sensor is its biggest disadvantage. The smaller the sensor, the higher the noise. The smaller the sensor, the larger minimum DoF.
There's nothing "holy" in DSLR (FF or APS-C) form factor either. In terms of sensor size, these types are just two points in the continuum from digital medium format cameras to cellphone cameras. However, there is an expected level of attributes for each camera size. For example, DSLR's have optical viewfinder which sees "through the lens". No u4/3 has an optical viewfinder that sees TTL.
