Yeah, sure, that will be true as soon as dragons really exist and your campaign world becomes a real place.
What you encounter in a sandbox campaign is what the DM pre-determined you would encounter without reference to your ability to encounter it in a fun manner. Had the DM custom designed the encounters, there could be a complete absence of suck (or as much of an absence as the DM's skill allows). But he didn't because then it wouldn't be a sandbox.
So, encountering the suck is inevitable, and the whole point of a sandbox campaign is to ring the PCs with the suck so that they can have the fun of learning not to step in it.
At least that's how a lot of the major proponents of sandbox campaigning define it.