I routinely use microscopes at work and have done a fair bit of yeast cell counting. I also culture my own yeast at home for brewing and I have used at least two of these cheap $100 amazon microscopes that I borrowed from friends. Neither I nor they think that those cheap microscopes were worth the investment because they work so poorly.
Here are some of the major drawbacks to those cheap microscopes. They tend to include some combination of the following:
-Flimsy mounting, so with any movement or vibration, the field of view shakes and you lose your place counting.
-Tiny field of view, so if you are not accustomed to using a microscope then you may have a difficult time finding the focal plane and you have to adjust the stage constantly while counting.
-Stage controls can be sloppy or tight, so you can easily lose your place on the slide/hemacytometer.
-Low quality lenses and mirrors, which make focusing difficult and can result in some parts of the field of view being in focus while other areas are out of focus.
-Uneven or Inadequately adjustable lighting, which makes it difficult to achieve the contrast you need to count cells that are not stained with any dye.
Although I have the luxury of taking my samples in to work to count them on a high-quality microscope, I don't bother. When you get the hang of culturing yeast you'll find a microscope is not necessary.