In addition to softness, pay attention to what sort of distortion you see out of zoom lenses. I don't know that there are any zooms available that won't show some sort of distortion at their extreme focal lengths, but when you're looking at a good zoom lens, it'll typically show only one type of distortion at a time, which is reasonably correctable in most post-processing programs. Superzooms or lower-quality zooms sometimes show complex distortions, though, and these can be trickier to deal with.
Less distortion to begin with, of course, is really much better, and this is made more difficult as zoom ranges increase. As another huge generalization, it's easier to make a high-quality prime than a high-quality zoom, and the larger the zoom range, the harder it is to maintain quality. You've got to be careful to compare apples to apples, of course, because there are some fine high-quality zoom lenses out there, but these lenses tend to be quite expensive because they're complicated and the optics need to be incredibly precise.
You might also consider what sort of lens speed you need. If you're shooting with natural light, you're probably going to find that a lens slower than f/2.8 is going to struggle quite a lot with indoor lighting (which could be a factor for weddings). This is another one of those factors that tends to be pretty rare in large-range zooms.
Looking at this from another point of view, if it were possible to make a lens of that range with excellent quality, I'd expect Canon to offer an "L" lens with that sort of range and speed. After all, who wouldn't want to have all that range if it didn't mean giving up something else in return? The closest Canon comes to that sort of range is their 28-300 f/3.5-5.6. This is an "L" lens that'll run you about $2500, but even at this level, if you go look up reviews for this lens, you'll see that it's a compromise compared to primes or smaller-range zooms. Note also that this isn't an especially fast lens, at f/3.5-5.6.
All of this suggests that Tamron's 18-270 lens is far from a lousy lens; on the contrary, it's remarkable that they can pull off that zoom range with any sort of quality at all, but an all-in-one lens entails a lot of compromises. If you can possibly cover that range with two or more lenses, you'll very likely be better off.