There is no definite answer to that possible, I think.
However, in my opinion, the impression I form from the books is that magic, while basis of the wizarding world [which in itself is a rather small in terms of quantity of wizards] was not that easily seen in the mundane world, except when dealing with the result of miscalculations or malice by wizards.
If we consider what we see described about Arthur Weasley's work, it is mostly to do with amok spells or magical items created by wizards. Yes, there are mentions also of wild magical creatures accessing mundane world, however, my impression was that just like ordinary animals, magical animals are more frequently avoiding humans.
And, starting from the second half of 20th century, and moreso now, I think, if magical creatures were not subtle enough, any massive outbreaks of attacks on humans would provoke investigations and some sort of mundane dealing with the magical problem, simply due to (population) size differences.
Dementors or other indirectly harmful magical beings might be subtle enough to affect lots of people without noticing, however, if we consider depression as magically induced, while substantial portion of people are affected by it, it is not as if we are completely helpless about it.
Additionally, while any particular magical creatures might be immune to some sort of human weapons (bullets, etc.), none of them in the JKR world seem to be immune to EVERYTHING.