This is very bad for a different reason. It makes it impossible to ensure the size of your text is readable for the user. Your source for this information is the font size of the root element. If you override that using a fixed unit like pixels, you discard this information.

Is 16px too large? Too small? How far away are your text size choices from being readable? You have no way of knowing because you threw that information away and decided 16px was good enough. 16px is not good enough for a lot of people.

The only way to ensure that the size of your text is readable for the user is to use a unit that is derived from their preferences somehow. That means that if you set a font size on the root element, it needs to be a unit like ems. Pixels break this mechanism completely.

Really, just give up trying to make ems work like pixels. You can’t, and everything you do to try to force the issue hurts your design. Design your text to be readable first and foremost. That’s its entire purpose for existing; to be read. If you can’t answer the question “is my body text a size that is readable to the user?”, then it’s bad design.