Thats true for most fish, but not all -- sardines have much lower levels of heavy metals, and are gererally more sudtainable. Canned sardines have been a cheap, easy way to add health fish to my diet, with (relative to salmon, etc...) has minimal environment impact.
It's of course true that eating apex predators like tuna is worse than eating sardines, but by fishing sardines you're greatly reducing the available food for species that eat sardines. The fundamental problem that we take out too much fish remains. That's also why krill oil is only a little better than fish oil: krill is an important part of the food chain and we're fishing too much of it.
What kind of problems do mean? Environmental? Or health-related problems?