I've had no operational issues. I have not noticed any audio issues, sounds fine with my low (32 ohm) and high impedance (300 ohm) headphones.
But it is not stated on the product page, nor the manual, that it is mono only in. Both 3,5mm and 1/4 inch input jacks have only Tip and Sleeve contact points. It does not sum left and right with a stereo input - it only amplifies the tip (usually left side) of the input.
People looking for a headphone amplifier for their stereo pedalboard (like me), look elsewhere.