Most policies only cover you for unforeseen illnesses. If your bunny has a pre-existing medical condition, your insurer wouldn’t cover you for treatment related to it. To get cover for this, you’d need to take out a policy that includes pre-existing conditions, which will be expensive.
Costs for routine treatments like vaccinations, spaying/neutering, worming or flea treatments also aren’t covered. It’s common for dental problems to be excluded too. Illnesses preventable by getting your rabbit vaccinated aren’t included either.