Her force-fields are powerful and I say a case could be made for those. But I'm thinking more in terms of her power over invisibility.
It hasn't gotten much focus compared to her other power in recent years, but the times it does it's been very impressive:
There was the story in Waids run that Galactus would seek her out specifically her because of her power to see and reveal invisible things. It's pretty big for a cosmic being to seek you out specifically.
Her recent increase in control to the point where she can change the colors of things. Which sounds small, but the potential for other applications coming from that can't be ignored. She can even absorb and trap light in field, which is an impressive feat when you think about it and goes beyond the normal scope of invisibility.
That's not to mention the growth in how much she can turn invisible/visible. In Kirby's run she was able to turn a cliff invisible, Byrne's it grew that she could cover all of Manhattan island, and it grew even more in Millar's run where she was able to turn something the size of the Moon invisible.
If any power of Sue's qualifies as having an "undefinable limit", it's her power over invisibility. It just continues to grow in both power and precision.