Even if what this person say is false, there should be a special or something able to break the shield. It shouldn't be 100% invincible.
No, it should. The usage of the Bubbler would plummet if something broke it. Plus it really isn't overpowered, just gotta keep your distance, bait the Bubbler if you need to, then get back a little and shoot, because you get knocked back pretty hard when hit while using the Bubbler. Makes it hard to get around and aim, honestly. The worst thing is when you get hit by the Killer Wail while Bubbling. A guy knocked me off the stage because it kept pushing me back, and back, and back.... I don't think you can squid while getting knocked back either (at least in my experience, I don't remember being able to), which means if you get wailed into a wall you're pretty much trapped.
To sum up my blabbering, when someone Bubbles, use your special as well. Unless it's the Echolocator of course.