I'll be honest upfront - Nirvana in Fire is not my cup of tea genre-wise, I just dislike palace intrigue dramas.
I was told it also had wuxia elements and it kind of does, but they do not make up for how much I disliked the PI stuff.
It's mostly well acted (by a lot of older actors, I appreciated that), mostly solidly written (though the plot does feel convoluted at times) and the rare fights are ok looking. I think overall it looks a bit dated, but not bad. Just not amazing.
For me personally it just felt long and tiresome in places. It might've started (or at least empowered) quite a few drama tropes I am not a fan of - the sickly character with incurable health problems and a mysterious past, the moral message of patriotically serving the country or a cause even if it means your death, episodes mostly consisting of characters talking in rooms.
To me this did not live up to the hype, plain and simple.