About Windowed Filters

1. Windowed FIR doesn't have steep enough skirts and hence they sometimes let the noise come in.

2. The adjacent channel noise is aliased in because the filter is too wide in frequency.

3. The window functions have very slow transition bands, so if you make them narrow enough to cut out adjacent channels, they will significantly attenuate the signal's high frequency components.

4. The Q of the windowed filter (normalized narrowness of pass band) is linear with number of taps.