I did an A/B test between some real amps and their models using an Avatar 2x12 and a Matrix GT 800FX today because some people say when they do this it sounds "thin" or there's a "blanket over the cab".
In every case the model sounded as good or better than the real amp and there was no thinness or blanket. If you don't turn the Axe-Fx up to be as loud as the amp then, yes, it can sound thin or muffled due to Fletcher-Munson but get the volume up and it's pretty wicked but, damn, it's loud.
That bad news is my ears are still ringing. Holy cr@p I forgot how loud guitar amps can be.
In every case the model sounded as good or better than the real amp and there was no thinness or blanket. If you don't turn the Axe-Fx up to be as loud as the amp then, yes, it can sound thin or muffled due to Fletcher-Munson but get the volume up and it's pretty wicked but, damn, it's loud.
That bad news is my ears are still ringing. Holy cr@p I forgot how loud guitar amps can be.