I use the ADI-2 DAC FS at my PC for a variety of things. What is "worth the additional investment" is highly subjective. I don't have a Fireface UC to say if its worth the extra cost. I went from PC integrated audio to the Focusrite Scarlett 2i2 to a Babyface Pro to this. Each device was a substantial step up in quality. Knowing what I know now, and after living with the ADI-2 DAC FS for a good while, for me, absolutely yes without even the slightest hesitation. The only real complaint I have is not having enough parametric EQ bands to achieve the Harman target curve for the earbuds I'm using and slightly limited frequency adjustment on the B/T controls. For this reason, I actually do S/PIDF out from my motherboard to my Babyface Pro, apply 3 bands of PEQ, and then S/PDIF out into the ADI-2. Obviously this is not ideal, but for me those 3 extra bands of PEQ are a life saver. The Babyface Pro was required for my mic and mix minus setup anyway.
It's marketed as having 5 bands of parametric EQ but I really consider it 5+2 with the Bass/Treble controls. The B/T controls are just a more limited form of the 5 full bands of parametric EQ but it usually works out just fine. Figuring out and setting the EQ took me forever but was worth the effort. I use Equalizer APO with the Peace GUI to model the EQ curve before applying it and test the end result with sine sweeps and such until I get it to where I want. There are nuances to every piece of gear. I found Sharp the best DA filter to my liking but sometimes try NOS. For some reason I did NOT like the SD Sharp or SD Slow filters. I don't know if its a psychological thing but the SD filters sounded more "crunchy?" The higher latency is not noticeable at all.
The headphone amp is great but I found that it struggled too much with my 600 ohm Beyerdynamic DT 990 Edition. Its been a while since I ran that test but if I remember correctly there were issues in the bass at high volumes. I don't necessarily fault the ADI-2 as 600ohm is notoriously difficult to power and I don't know if it has something to do with impedance matching but this guy talks about that subject matter here (https://youtu.be/JpqnGbP9aTo). This is a non-issue for me as I don't use the DT 990 anymore. Sometimes you have to learn the hard way what you like and don't like and I found out that I didn't like open back headphones. Believe it or not I use the $10 Panasonic RP-HJE125-K on the phones out and they sound unbelievably great after EQ. Best of all they're cheap to replace and very comfortable in extended use. Be assured there is plenty of power for most headphones/IEMs/earbuds.
I don't always use earbuds/headphones so I use the XLR outs to my 2-ch amp and the RCA outs to my subs. The simultaneous outs are greatly appreciated. The EQ automatically switches accordingly. No complaints there but in my case I have to switch EQ presets on my Babyface Pro.
There is a bit of a learning curve to the GUI but once you figure it out its a non-issue.
The remote is very handy and some of the buttons are re-programmable to suit your needs. My only complaint is that its not backlit but that's nitpicking.
It works with RME's DIGICheck if you need it.
The ADI-2 DAC FS, for me, was a game changer and a perfect fit. I can't imagine not using this many EQ bands at this point anymore because (let's be honest) most headphones/IEMs/speakers are flawed no matter how much they cost. If you want to achieve a certain target curve you have no choice but to EQ. It comes down to whether or not you're willing to spend the money on the best PEQ+DAC+headphone amp solution in a box RME has to offer. I don't have a wall of retired DACs to compare it to but this is the best sounding DAC I've heard yet and I have no desire to try something else. There might be a better DAC or better headphone amp out there but the value of this overall package cannot be beat.
There are some on here that say you shouldn't need more than 5 bands of PEQ but I beg to differ. There's nothing wrong with more bands if it helps you achieve your desired target curve more accurately. I'd love to see 8 or 9 bands of PEQ plus slightly more flexible B/T controls (higher bass freq limit and lower treble freq limit) in the next generation ADI DAC.
Hope this helps.