Head phones are a great way to mix.

But, it depends on the headphones and how flat they are.
I mix everything using headphones - audio-technica ATH-M40fs.
They are great for mixing.

However.
When I've finished - gotten to where I think it is a "finished" mix...I then listen through studio monitors - Yorkville YSM-1.

Almost every mix needs some small amount of adjustment to get the best sound through the Yorkvilles - usually some bottom taken out of a few things and a bit of highs bumped in others.

Once I've done that and return to the headphones the improvement (from using the studio monitors as the final call) is obvious.

150+ Songs produced using BIAB RealTracks
And 50 more...
and some more...