Keys and synths can appear to be an easy task to mix. They have dedicated outputs and don't require microphones. This very fact is what makes them difficult to tame sometimes. Their EQ spectrum can be all over the stereo field and this can make them unruly when it comes time to mix. My approach is to save the keys & synths for last. I like to mute them and address the drums, bass, guitars, & vocals first to get a nice even balance. Then the keys are like the icing on the cake. They layer a nice thick sound over the mix and it allows you to use surgical EQ to get the pockets filled to your taste. Keys are sometimes the glue that pulls a mix together and sometimes they cloud up the mix and make it murky.
Approaching keys with care can give you more
control over the final balance of the mix. In some mixes, it would be a
good idea to add some light delay on a 16th note. This can give the
appearance of a wider sound for the keys in the mix. You can also split the signal into two mono tracks and add reverb to just the left channel. This will give the keys more depth in the low end and allow the high end to shine a bit more.
tracks typically benefit from proper EQ and a silky sounding reverb. The
mood of the track will lead you to what style and depth of reverb to
use. I love to hear a dark reverb with long tails, so I like to use a
low pass filter on the reverb return. This allows the initial attack of
the bright notes to shine with a very pleasing trail of dark tones
following behind them.