I'm not very sure of how suitably is using that construction in complementing others since its start has a very negative indication and may make the other person feels bad and/or uncomfortable.
I didn't find much about these two in Google search but when I have typed them in the box, the auto-complement suggestions were mostly negative for "You are nothing but..."
And the results for the "You have been nothing but..." were mostly positive.
...which doesn't make sense as those two constructions are very similar to each other. Thus, if they have been used in complementing, would they make a suitable construction for that particular field and leave a good impression to the other person?
Examples of how I want to use them:
You are nothing but gentle and kind.
You have been nothing but nice to your fellow members.