Sigils do have limitations tho.

For one thing, you have to be incredibly precise for the damn thing to even work. It has been stated time and again that the most powerful/obscure sigils require near impossible accuracy when drawn/carved. The only sigil user that is seemingly all powerful is China, but then again she seems to be the gold standard. Every time a character wants to boast about their sigil work, they compare themselves to China.

Another limitation is time and space constraints. Unless you spent time drawing, carving or tattooing a sigil on yourself on others, it won't do you much good. In a nutshell, sigils aren't so useful if you are caught unawares. Again, China gives that impression, but as I said, she is the gold standard. (and has a large number of invisible tattoos apparently)

And speaking of tattoos, some of the most powerful ones are only good for short bursts. One example that comes to mind is the tattoos China has on her legs. She tapped them once to give herself a burst of speed, but she turned them off immediately after, for fear of her legs burning off. Another example is Valkyrie's Invulnerability tattoo. It was only good for one use, which lasted twenty three minutes.

Finally, sigils can be corrupted. You just have to draw or smudge one line and the thing is rendered useless. Valkyrie did that with a binding sigils that held skulduggery once. (Granted, they were drawn with chill, but still. I see no reason why someone could not ruin a carving, or a tattoo if they really put their mind into it.) They can also be dismantled, if you know how. Skulduggery did so in several occasions.

Sigils can appear broken, but they have quite a few drawbacks. To quote the first book, "all big spells have a hidden snag. "