Hey man, upstrokes are actually really helpful to practice! If you play a song purely with upstrokes, it will actually improve your downstrokes. This is because even if you are only downstroking, you MUST make the upstroke motion back in order to make another downstroke... so why not actually put your focus into practicing upstrokes so that way your muscle memory develops faster, correctly, and also gives you more control over your picking overall. :)

All you need to do is play something like master of puppets with all upstrokes, and you'll instantly see the results after like 5 minutes of practice.

Some things this affects:
-string skipping
-sweep picking
-strumming
-alternate picking
-speed of playing
-control over your picking
-picking power