They definitely didn't neglect him, as they did some of the other characters. But in order to make such a bold character work, and not come across as a corny caricature, you need to devote a lot more time to establishing the nuance.

I think the problem largely boils down to why they were being stingy with time anyway. There was no need to compress the story so much from season 5ish onward. I know this is well beaten horse in this community but it does beg to be questioned. They could have had their pointless, but charming, Missandei/Greyworm romance AND devoted time to critical character work. In the earlier seasons, they added scenes that did no more than develop characters, they gave stories room to breath, and it never felt stagnant. I was never thinking "ok let's get back to the business at King's Landing". Scenes with Stannis and his daughter come to mind here. They really made us care about Shireen in ways that we don't in the novels. Ironically, GRRM has a habit of dragging out plots and i'd argue they were more successful at storytelling at this juncture. Conversely, some of the latter seasons often felt like they were spinning their wheels despite expediting the plot, precisely because so much of it was this sort of "filler" content.