an important thing about each immortality-seeking variant of Lucent is that attaining immortality is not the end of his problems, and in many cases just serves as a jumping-off point for him to have more problems
for Dr. Ashveil in particular, drinking Hesperidean Cider indirectly fucked him up in multiple ways. there's the whole survivor's guilt deal with his best friend's permanent death, obviously. but even without that, I feel like he would find that true* immortality makes it easier to justify working himself to death (sometimes literally). it makes it easier for him to justify doing things that cause quite a lot of pain and harm to himself. it makes it easier for him to justify holding himself at a distance from everyone else. it makes it easier for him to devalue his time, his body, and his mental wellbeing.
sure, he can go back to the Surface now, but why would he when his work is in the Neath, and his work never ends? what pleasure could he take in walking beneath the sun again, when sunlight itself has become hateful to him? when he knows how wrong, wrong, wrong the universe actually is?
I don't think immortality itself has to suck. I think he's someone who is capable of embracing both the joy of meeting and the sweet sorrow of parting, of forever finding new things to delight in. but "capable of" doesn't mean he's currently doing that. it's something he has to learn, especially as a revolutionary.
*as true as the immortality given by cider can be, anyway. we know from Sunless Skies that it's not so simple. but for the good doctor's purposes, it might as well be