Well, I rolled up a naga demigod, and had a few noteworthy encounters: wolves, a giant dingo, a human bandit camp, and some bogeymen. I tried to use my tail a lot, to get a feel for how it works out.
Overall, it seems like the tail is reasonably powerful against smaller enemies, when it hits, but fairly useless against larger enemies. I don't think that by itself is all that game-breaking, although the ability to one-hand pikes without penalty concerns me (see end of post for a fuller explanation).
The first wolf I engaged passed out from the first tail attack, although that seems to have been a fluke. The rest were at least able to withstand a few hits, but still seemed pretty easily damaged by the tail. However, I probably could've finished them faster if I'd gone for paw stabs with an iron spear, followed by a spear to the brain after they dropped prone. And it's not like wolves are hard for demigods - in 0.31, I usually played elven adventurers, and I was always happy to see wolves early on: they're easy to kill even with the starting
wooden spear.
The giant dingo was the next major encounter, and seemed to basically shrug off tail attacks. My tail wasn't strong enough to cause any significant damage, and it wasn't a good grapple tool, either - the giant dingo frequently broke my grip. I think in that case, I would've just been better off using a spear and another weapon, and hoping that the spear lodged itself in the giant dingo's body (which, IIRC, is what I ended up doing, once it was apparent that my tail sucked against the beast).
The human bandit camp seemed to have gone smoother than normal, more like what I'd expect from the third bandit camp or so. I think they suffered nerve/bone damage quicker than I would expect for a freshly-made adventurer. So, it's easier, to be sure, although I'd describe it as demigod+ rather than gamebreaking. I think the main circumstance where it would REALLY matter is if you're facing a bunch of archers (you do NOT want to give them time to reload), or a powerful bandit leader (they dodge like crazy compared to the others, so if you're impairing their mobility on the 1st hit instead of the 2nd, you're less likely to suffer nerve or bone damage). I'd have to play with it a bit more, to be sure, though.
The bogeymen were as frustrating as normal, dodgy little bastards they are, although at some point I entered a martial trance. I tend not to play dwarven adventurers (elves were faster in 0.31, and humans have more fun loot in this version), so I don't know how the trance compared to normal. However, I think I still died about as quickly as I should have, given how recklessly I was attacking the bogeymen.
Also, I'll note that I tend to max strength and agility, and I got a bit over 1900 speed - high but not unheard of for an elf in 0.31, and sorta high for a human in this version.
So, my overall thoughts are that the tail attack IS a bit powerful, but not powerful enough to break demigod adventuring. At most, I think it would just make the early game a bit easier. The tail seems to be fairly useless on large creatures (and I bet it would be a bit weak on tough skin/armor as well), which are one of the main sources of challenge for demigods. You probably will be a bit buffer against early enemies, but really, I don't find most of those to be that challenging for a demigod anyway (with the exception of archers, who need to be dropped ASAP). Megabeasts, heavily armored fighters, giant animals, and the like are probably still going to be an appropriate challenge. I will note that the tail attacks seemed like they were causing nerve damage, chipping/crushing bones, and ripping off body parts a bit more than seems reasonable for a constricting tail; but, I did end up holding a severed wolf paw in my tail, so you could easily make the argument that this particular "bug" leads to enough awesome things to be worth keeping
. Now I just have to see if I can rip off a head, rather than just jamming the brain in...
One-handing pikes, on the other hand, sounds iffy. My experience is that piercing weapons in particular benefit from being able to be one-handed, because they have a tendency to stick in the enemy's flesh. When that happens, I've found that you get a hefty bonus to hitting with other weapons, AND if you have another weapon handy you don't have to waste a turn removing the one that's stuck in your enemy. Add to that the fact that spears are generally recommended as anti-megabeast weapons, and it seems like one-handed pikes has the potential to have a long-lasting impact on game difficulty.
Also, I should not that I didn't fight any significant zombies, nor any horses, both of which might be worth testing for size reasons. Horses are nasty because they charge you, move fast while fighting, and their hooves have a tendency to cause permanent damage - so nagas' increased size might make horses less deadly than they should be. And zombies, well, size seems to be important there. I've found that camel zombies are pretty nasty, and humanoid zombies are fine as long as you can kite them properly. I don't know if being a naga would significantly impact either of those, though.
So, as far as demigod is concerned, I'd say that the tail attack is powerful but not too OP, but one-handed pikes and increased resistance to being trampled might being a bit unbalancing