Yes, Star Trek went overboard on the "Forehead Aliens of the Week." Okay, budget is a concern, but there are ways around that. Instead of giving us new Forehead Guys every week, re-use the same Forehead Guys. Really, Voyager and Enterprise could have benefitted from using their original aliens like the Hirogen or the Suliban more often.
For that matter, when they were constantly visiting races who looked exactly human on TOS and TNG I think they should have re-written so that instead of seperate race these people were humans from colonies that declared independence from Earth and the Federation. I know that episodes like The Vengeance Factor or The Hunted (episodes I was watching when I first thought this up) would still have been the same episodes if they were human colonists rather than seperate races.
CG aliens would have been tricky duirng the period Star Trek was on TV. And really, I thought the CG Gorn on Enterprise looked worse than the lizard suit in Arena.