A multifunctional or convertible chair is generally less comfortable to sit on than a dedicated armchair because it has to hide the mechanics that help it to extend into a bed somewhere within the frame. This means the seating has less spring and will tend to feel firmer to sit on than a regular armchair.
But that's not to say they don't still make great accent chairs and a sleeper chair could still be used as your main seating as long as you don't mind sacrificing some squish-factor.
When it comes to sleeping, how comfortable a sleeper chair is depends a lot on how firm you prefer your sleeping surface, and which type of mattress you choose, with the thickest depths often offering a comparable sleep surface to a regular bed — albeit potentially a little shorter or narrower than a standard mattress.