A major downside to making a world in which both monsters and magic are real is to what extent it is hidden from the world and the means for doing so. If you can do whatever you want and just wipe the minds of mundane people after, the system is obviously a) going to be abused and b) make your characters into jerks. To say nothing of being difficult to sustain in a modern age of high-tech phones, google maps and so forth.
For the Ghoulish novels there are two systems in place. The family of magicians who are called the Illuminati [this is both an insult and the name they've picked for themselves] can alter memories and make illusions and such. This is not always effective, and sometimes they need to have humans killed, hard drives stolen and so forth, but they try to hide that reality from other magicians in order to preserve their image as able to fix any mess. The reasoning behind this in-universe is that magic is quite dangerous and having more families of magicians would lead to even more magicial fallout(which creates monsters, urban legends, ghosts etc.) and screw the world up further. Also, magicians don't want to share power. But the stated reason for the system does have altruistic aspects.
(There are hints that repeated mind-wiping and the like has damages people via increases in ADD and the like, but I'd rather leave that as hinted at since it's a giant minefield I have no desire to dance around in. It is probably not true but does make for a fun conspiracy about a conspiracy.)
The other system, that being fae glamours over other monsters, is less well understood. The fae offer the service, and seem perhaps compelled to do so, but no one knows why and they aren't likely to tell. As the fae are a mystery even to the magicians this is likely going to remain entirely unsolved during the series, along with many other questions about the nature and goals of the fae. Technically, the first system could get by without the second but it would definitely be very difficult in the modern age.
Criticisms and worries of the systems will crop up in the novels but for the most part, more magicians does mean the world would be far, far more dangerous place so the general conceit behind them hiding magic and monsters from the world is, if not necessary, at least understandable.