Onyx is nothing more than a GUI front end for built in UNIX commands. IF you know what you're doing, it can be helpful. For the average user, who is not a UNIX guru, you can end up doing more harm than good.
You should not need any 3rd party utilities at all to "clean" or maintain your Mac... That said, as I have already pointed out, Onyx is technically just a GUI that lets you easily access the built in UNIX tools... It is not dangerous in the same way that the scam apps like MacKeeper and CleanMyMac are, but it can still be dangerous.
An electric carving knife can be a fantastic tool when it comes to carving up the holiday turkey. You would not, however, let your 2 year old, who doesn't understand what it can do and how dangerous it can be, take it in hand and start slicing things up...