I believe we can have a fairly approachable extent to what Dracthyr capabilities might reach. We don’t really have an exact source, but let’s say it would start from different points:
- What mortal races was Neltharion using to give them their, let’s call, “determination”.
- The fact that they are also dragonkin.
Going forward from these two points in particular, we should think about the time they were made. From what is told in the cinematics, this race was created - at least - 20.000 years ago, so it’s fair to say that their mortal side belonged to the mortal races prior to their encagement.
What this time-gap means is that, essentially, there were no humans, dwarves, nor elven races at this point as for yet. The only mortal races there could be in Azeroth, particularly in the continent of Kalimdor, could be Trolls, Yaungol and then some, which were not developed in magic.
I might be showing this as a flaw, but it really isn’t. We still have to think that the Dracthyr have a portion of the Aspects themselves within them which is not to be catalogued as weak magic. In the terms you’ve put (Arcane magic), this would technically mean they can tap into the ley-lines far more instinctively and easily than any other mortal race, maybe tying up with humans which are known to be a race with ease and enormous potential for magics.
Now, after that ramble, it is really up to debate. I’d say they are not as strong as a dragon on average if we speak on raw power (I doubt any dracthyr, from the control they’ve been under, ever had the chance to grow their power up to what Malygos or Kalecgos could’ve ever had), but as for capabilities I’d personally assume they are just as good, if not better, as your average human mage.
I would clarify though that Dracthyr are - made - to be soldiers and menders, which causes them to be, on average, the strongest mortal race there is (at least that I remember). They know what their powers are and were born for that purpose alone. But they were not enough to stop the primalists on their own (controlled by Neltharion though), which already places them a few steps behind the mortal races of Azeroth. Regardless, Dragonflight will put their freedom to the test, so I’d think they are really in equal ground with the rest of mortals.