the fact that somebody can break in means that somebody can break out too, with help from the outside

There is the issue of wands. Wandless magic is, at least for humans, much less powerful than wand magic. So it's perfectly conceivable to me to have obstacles that are virtually impossible to overpower if you're wandless, but possible to overpower if you're a wand and are a good wizard (which Voldemort surely is).

The "his followers could continue to communicate with him" is indeed a real problem. But it seems (both in canon and in MOR) Azkaban itself is not so hard to break from the outside, only (almost) impossible to escape from inside.