Curse skulls used to be stationary enemies, but had higher AC (and maybe health?) compared to current curse skulls. It is difficult to make stationary enemies interesting; a lot of slow enemies have enough trouble posing credible threats, and infinitely slow enemies have even greater trouble. So I get why curse skulls were changed from their previous version, but I don't get why they were changed to their current version. I would suggest the changes don't fundamentally fix the issues that old curse skulls had, and yet introduce some new deep problems.
How do these new(ish) curse skulls work? At present, a curse skull will not move when it is in LOS, but once it is outside of your LOS it will stalk just outside your vision, kind of like wandering mushrooms. Except curse skulls seem to be even better/more adamant about staying right at the edge of your LOS as you travel across different terrain, so that regardless of corners and doors and whatever else you try to use, the curse skull will basically always be positioned such that, if you take even one step back from the direction you were exploring, you got a curse skull 7 or 8 squares from you. (With good stealth you can lose it, but it is very adept at tracking and maintaining distance otherwise.)
Despite all their flaws, what fully stationary enemies, including old curse skulls, can offer is a sort of static puzzle to solve: A map exploration problem. Basically you can think of stationary enemies as elaborate traps, part of the terrain. Now you must either disarm them at danger to yourself (by destorying/killing them), or work around them some other way. Whether one thinks this is a good decision or good design is up for debate of course, but that is what stationary enemies can offer. By being semi-mobile in their strange way, curse skulls lose this.
And yet they don't get the benefit of mobile creatures, as they remain still when in your LOS. Thus, the moment when you are in LOS of a curse skull, and thus the moment when a curse skull can potentially do anything bad to you, is also the exact moment at which it has precisely all of the same problems the old curse skull had, in terms of tactical one-dimensionality. It can only hurt you when it is in LOS, and it cannot move or reposition while in LOS. In terms of how you approach an encounter tactically, the new curse skull isn't doing anything fundamentally different from the old one, and suffers from all the problems of a stationary foe. (The "fight/flight" problem is solved with one key stroke unless you teleported or were shafted or whatever into the stationary enemy's LOS; you lose the dynamics of repositioning in melee, which is a shame.) So, in terms of fixing the problems of stationary enemies, the new curse skulls seem like, at best, a lateral move. And yet it loses the one distinctive novelty of the tough stationary enemy archetype, as well.
The above was a more abstract comparison between design problems and goals of two types of enemy. But what about the actual game play of fighting a curse skull? How does that play out?
Unfortunately: Very frustratingly, in a bad (not challenging/interesting/good) way. So much so that despite all of the above, this is the main problem. By far.
The issue with having a wandering mushroom that spams undead summons and torment at you from range is that every time you take one step backward (relative to the direction you were traveling), for whatever reason, the curse skull is right there and exudes some wraiths or takes half your health, at which point you take one step forward and mash five. This basically happens until you carefully plot some trip back upstairs without backtracking, or just use fast or teleportation. If you bump into the skull again, rinse and repeat. Usually you just skip the level (at least I do).
When you come across a stationary enemy in Crawl it puts an auto-exclusion down, and it makes perfect sense for Crawl to do that. The curse skull is basically a stationary enemy, just one that is always positioned just "behind" you, outside of LOS. You get no auto-exclusion for it, however, and yet it would make perfect sense for you to get one, for all the reasons oklobs and roxanne and statues get one. Thus autoexplore, autotravel, autofight, incidental backing up for whatever random reason (oops walked past this gold/item), misclicks, etc. that would land you in curse skull zone are not blocked and/or warned against, the way they would be if you tried to autofight something that was within the penumbra of, say, an oklob's exclusion radius. And yet you, the player, knows that the curse skull is exactly there. You just have no good way to inform your little @ avatar of the information you possess. I don't think it is fun to punish auto<foo> and incidental backing up, and I don't think it is good that the only way to make normal tools like exclusion work well is by placing new exclusions manually behind you every time you take one step forward.
Now, again, I think I can get a gist of what the intended goal was for new curse skulls: With a curse skull following you, your retreat options via movement are greatly limited, and in particular, you will not really be able to turn back the exact way you came. I can see how that idea has appeal in theory. However, by the time the skulls show up, you will absolutely have ways of circumventing them, and you can always just carefully trace your way back to the up staircase when you first see a curse skull (assuming you didn't land close to one from shaft/tele). So not only does the "blocking retreat" effect also cause lots of bad movement/interface hassles, but also even the intended, possibly interesting/good effect they might have, doesn't really impact you by the time you find them.
Having lower (but still high!) AC doesn't make them much less dangerous, because, well, they still have high AC, omni-resistance, really solid health, and they cast torment and spam undead at you. But now it is much more difficult to use terrain to get in close fast, because they intentionally try to stay just outside of your LOS, or right at the edge. So basically you either use a few select spells/consumables to close with them and bash with a good antimagic or holy wrath weapon, or blast them with dispel undead. In my experience, if you can't do either of those things, you don't touch them and use staircases to tiptoe around them while exploring the level, if possible. If one (or heaven forbid, more than one) keeps popping up I just skip the level.