It's not their speed, which is not only appropriate to their size but also proportional to the speed of the other weapons in the game versus their weight. It's not the damage, which is also appropriate considering the increasing returns on AR and the speed of such weapons. In fact, the speed/damage/range ratio, in other words, balance*, is not significantly different from that of other weapon classes. Katanas are very fast but have mediocre damage and range. GS are well balanced between speed, range, and damage, Spears have incredible range and decent speed, but low damage, and UG hammers/axes have high damage, above-average range, but low speed. Judging by this balance approximation, UG weapons should at least be viable in PvP. From here on out, I'll count out UGS**. They're the most viable of the UG's and see decently frequent use, especially in past PvP. I don't personally consider them broken, and one can use them to great effect even in high-skill contests. This thread really focuses on the axes and hammers. After a large amount of thought on why these weapons are so difficult to utilize, and how they could be improved, one thing came to me. Poise break value.
Rather than changing it to a debatable number, I think it should be eliminated for these weapons. In other words, they should stun every time they hit, regardless of opponent's armor. A slow weapon may be one thing, but if the opponent can slap on Giants, even Giants without the helm, and be able to take a hit to stunlock the UG user before he can get in a second swing, the weapons are nearly unviable. The fact that their movesets are easy to parry and bs can be overcome by skilled play and fighting unlocked in the case of bs, but the ease of use of heavy armor and the existence of the 76 breakpoint means that UG weapons lose their one decisive edge. Add in the DWGR to the opponent's repertoire, and UGW users are outclassed in dodging ability while being forced to wear the heaviest of armor to break even with the breakpoint their opponents are sure to be using. The point of instant stun is not to make them able to stunlock the opponent to death. With the speed of these weapons, one can roll out of their stun without even trying. The idea is that by being able to interrupt their opponent's attacks, trading hits while using a UGW is a definite advantage, and makes the use of these weapons about timing and interrupting an opponent's pattern. This should be coupled with a DeS style system in which the user of the UGW gets a poise boost while attacking. This way, the options of skilled players facing these weapons are greatly reduced, and the UGW user actually has some control over the fight rather than waiting for a huge opening while evading the enemy's attacks.
Keep in mind this is just my humble suggestion, and feel free to discuss ant point out flaws in my argument, as there are a lot of people here who know a lot more about PvP and the use of various weapons than I do.
*This is an absolute generaization. Many more things contribute to real balance, like weight and stamina cost. I refer to the general parameters of each class, and am aware that individual weapons vary, though I would argue that moveset is just another application of damage, speed, and range.
**As well as the BKGA. I would consider its balance to be broken, as the power and range are similar to a weapon of its class (both very high) while it's speed is two classes faster. (around GS). It does not need to be buffed, if anything it should be nerfed but that's a different discussion.