The trouble is, a lot of things labelled as "monitor" are pretty ordinary speakers with a label to make them more expensive.
However, assuming "proper" monitors, they do make your mixing easier (if not better). The whole idea of a monitor is to give an accurate idea of what your tracks really sound like rather than make them sound artificially good. Lots of "ordinary speakers" throw in things like bass boost or a presence bump to make recordings sound artificially pleasing to the ear. If you mix so things sound right through a speaker like this, somebody with more accurate speakers will think your mix is pretty flat.
You CAN learn what things need to sound like on "ordinary" speakers to sound okay everywhere. It's a matter of doing a trial mix, listening to it all over the place and making notes, then revising the mix based on your notes. You'll eventually get there. Indeed, even with the best monitors, there's a learning period while your ears get used to the sound. This is why my main studio monitors are currently 32 years old. Heaven help me if I have to replace them!
A couple of points...first, although most things sold as monitors these days are indeed self powered, there are a lot of good monitors that aren't and need an amp. This isn't necessarily a problem.
Second, I fully concur that there are some good hifi speakers that are also excellent monitors. Indeed, some of the offerings from companies like Rogers and KEF sell the same speakers for both, usually just with different input connections. However, don't be fooled by some of the ridiculously priced Audiophool units.