Of all the things I do as a plumber, clearing a clogged drain is my least favorite job. That being said, when I'm not busy with the vintage plumbing jobs that I prefer, I'll clear the smaller-sized drains myself and refer the larger drains to rooter services. My customers expect it from their plumber and appreciate the service.
Most plugged drain problems are caused by food waste, hair and the occasional foreign object. When I clear a drain I'm only concerned with running my tool in as far as the three or four inch pipe. I know that the water flowing from the toilet will keep the larger pipe clear and scour out whatever I have pushed into the line.
But I draw the line at de-rooting large soil drains. Those are the large drain pipes that run from the house to the sewer. I consider cleaning or snaking these large drains to be a specialty. It's not a do it yourself plumbing repair project!
The main things that cause sewer drains to become stopped are roots and pipe failures. One will lead to the other.
In the summer, when it's been dry for quite a while, we can always be sure to get several calls from homeowners with tree roots clogging their drain pipes. That's because the tree roots seek out the water in the drains.
I don't own, and never have owned a machine suitable for clearing the main drains... with their 3 to 4 inch pipes. These drains take an industrial-size drain snake, which I don't have.
No, for clearing a large-sized drain or a blocked sewer drain, my favorite tool fits into my wallet... a business card. I'd much rather have the number of good drain tech that I can refer.
Now I've told you what job you can't do. But you can clear a shower drain yourself.