The RME ADI-2 is an excellent AKM449X implementation, and for that sound it can certainly compete with some more expensive DACs. You may find it a little cold, analytical, and dry, but that really depends on your own subjective calibration point. Others won't perceive it that way, and there are other DACs that are even more so in that direction on the spectrum.
Whether or not the headphone output itself is powerful enough to bring out the best of your headphones will depend on the headphones you're using, but it is quite capable.
The biggest downside is the relatively complex UI and customization options. Most consumer DACs and headphone amplifiers have a few choices and a volume knob. The ADI-2 has a whole lot of settings and a bit of a complex interface; you could easily (mis)configure something and not know it or have a hard time undoing it.
Whether or not the headphone output itself is powerful enough to bring out the best of your headphones will depend on the headphones you're using, but it is quite capable.
The biggest downside is the relatively complex UI and customization options. Most consumer DACs and headphone amplifiers have a few choices and a volume knob. The ADI-2 has a whole lot of settings and a bit of a complex interface; you could easily (mis)configure something and not know it or have a hard time undoing it.