It is normally worn on the ankle because it was found that some people can manipulate their thumbs sufficiently to slip it off their wrist. If the tag is used on women offenders when they're pregnant and their ankles are prone to swelling up, there is a wrist version of it.
The electronic tags used in Britain can be cut through with a pair of strong scissors. They can be removed very easily. We do that because of health and safety considerations.
Some people rightly question whether there is sense in that, so the GPS tags that have been used in the current GPS pilots and for the pilot in the National Health Service are a different sort. They're not a plastic strap with an optical fibre, but a leather strap with a steel ring, and it would take a pair of industrial bolt cutters to cut through them. First of all your offender would have to get a pair of industrial bolt cutters, and it would still take him or her 20 minutes to cut through them, which would be a significant delay in breaking free of the tag.
There isn't a simple and single answer to your question on how easily they can be removed because you can make tags and straps in different ways.