All PMI policies will limit or restrict benefits in some way, and you should be told how. These limits may apply to the cost or type of treatment, the length of treatment or where it takes place. You should understand how the insurer will deal with any pre-existing medical condition you may suffer. If you decide to move between insurance companies, there may be implications for the cost and level of your cover, and these should be outlined to you. Medical inflation rises faster than the standard rate of inflation, so it is likely your premiums will do the same.
Policies vary in the types of treatment they cover.
Also important are the cost of claims paid by your insurer throughout the year. If this rises, your premiums could too. The size of your excess will also affect premiums. Your policy may have an excess, that is, the first part of the claim, which you have to pay yourself. This may be automatic, but it may be voluntary and the larger you choose to make it, the lower your premiums will be.