It's a risky choice, unless you're very good. You should go in with your eyes open, knowing that you'll be lucky to get an academic job afterwards, especially if you're in a 'pure' field without many applications. For industrial jobs, you'll probably have to re-train through self-study. After a maths phd, doing so may come easily, but it will also take effort.
You'll spend three to seven years (dependent on your location and ability) working on an extremely specialised topic. There is no guarantee that you'll finish: circumstances beyond your control like your relationship with your advisor can derail your progress regardless of your ability. There is also a 'point of no return' after a year or two where it becomes very painful to cut your losses and walk away with nothing.
Doing a maths PhD could open some interesting doors. It will improve your employability, but it's an inefficient way to do so. The only good reason to do one is that you really like your research topic and want to spend years immersing yourself in it.