You can't do anything else in that time. "No interruption" sounds OP but you can't cast any spell since you're already casting town portal. And it's not an immunity bubble, it's just 50% reduced damage. But in that time of standing there, your opponents can unleash hell on you. Not sure why you think this is gonna be super OP in PvP. If you stack crazy health regen maybe, but then again - if you're regenerating more than what's half of your opponent's unhindered damage, you're OP without that bubble, too.