Ah, the return of the "You can't do a thing despite a thing being an explicit example of what you could do" argument.
I believe the argument against using Dhar is more along the lines of:
1. While Mathilde is immune to physical Dhar corruption, she's not immune to metaphorical Dhar corruption. Namely, that using Dhar requires cultivating a certain mindset, just like Ulgu, and that mindset is a pretty evil one.
(Personally, I feel like that's not really something that can't be overcome if needed, but still, best be careful)
2. Using Dhar is an explicit breach of the Articles. This is bad because
everyone is likely to turn on her unless it's for an
extremely good reason (like taking down an undead army threatening a whole province or Karak good reason), and because for Mathilde such an explicit breach is a pretty serious blow to her internal ethics. And even
with a very good reason we will still have people clamoring for her head.
(Personally, I suspect that the rest of the Grey College LMs might let us dodge the blame if it's important enough and we clear it with them in advance. But it's still extremely suspect.)