It depends on the situation. I think most of the Terran players who complain about Ultralisks are Bio players. Their armies have low burst, and they don’t tend to make many Marauders if they can avoid it. Even though Terran has tools that can handle Ultralisks, it isn’t easy or cheap to transition into them from Marine-heavy Bio.
Either way, it is ridiculous to expect Ultralisks to be powerful enough to storm a large line of Tanks. Tanks are the Terran counter to Ultralisks in Brood War, and they remain an effective counter in StarCraft II as long as their supply count is large enough for the range and burst to take effect, or as long as they have something durable (walls, rocks, terrain, Hellbats, Thors, etc) in the way to stall for extra time.
The main advantages of Ultralisks are their high armor, speed, and splash damage, but the armor and splash damage advantages are negated by the Siege Tank’s own burst and size or spread respectively. When there are enough Tanks in an area then most of the Ultralisks won’t be able to get into range without help, and they don’t need to be able to do that either because Zerg (like all three races) has plenty of other tools to help break Tank lines.
There are counters for everything in the game. Ultimately, it comes down to using your units properly, and having them ready (sufficient numbers, right place, right time).
Small amounts of units can beat Battlecruisers, Carriers, or Thors too. You don’t need a massive supply or resource lead to trade against these units unless you are trying to beat them with the wrong thing.
The issue you might be complaining about is that Ultralisks do not scale up well into larger engagements, but that is a result of Ultralisks being a bulky melee unit rather than a problem with any of the Ultralisk’s stats. Melee units simply do not scale up as well as ranged units, so at some point you need to supplement them with something else, or you need to keep the fights small (such as by attacking multiple locations).
In terms of raw numbers, sure. Ultralisks have more potential counters than Thors, Carriers, or Battlecruisers. However, I wouldn’t consider this a problem.
Ultralisks have the mobility advantage against most of their potential counters. They can avoid fights or attack other locations if needed. Ultralisks can also wait until a Fungal, Abduct, or a Blinding Cloud is landed before taking an engagement, and that will turn the tables on most of the Ultralisk’s potential counters. Thors, Carriers, and Battlecruisers are usually in the opposite position. They have a smaller list of potential counters than Ultralisks, but almost everything on that list (and almost everything that they are good against) can outmaneuver them.