Mechanics are incentivized to cut corners. Most mechanics are paid per-job, not per-hour. The quicker they can get the job done, the more money they make. You as a shade-tree mechanic are incentivized to perform the work correct. As long as you have basic mechanical skills and the shop manual, you can do a better job.
Every time I've taken my car in for work, I found places where they cut corners. A biggie is mechanics almost never use torque wrenches for precise torque of bolts. They are always overtightening gaskets. That is why after you take a car in for work, frequently you end up with a leak. Another is failure to re-install all the little plastic covers, especially underneath the engine.
My first car after college was a Honda Civic. The Civic requires a valve adjustment every 15k miles. Sure enough around 10k miles the valves started rattling. I took it in for the 15k service. No change. Went to another dealership and complained ... paid to do it again .... came out worse and now the valve cover was leaking oil. At 30k miles I decided to do it myself and purchased the shop manual. First thing I learned was the valves must be dead cold to perform the adjustment. The engine needs to sit overnight before the adjustment made. This requires the service center leave your car overnight in the bay, so it is ready the next morning. Both times, I dropped the car off in the morning and picked it up a few hours later. There is no way they let it sit the required time. Then I discovered the valve cover was torqued WAY to hard and the spark plugs were full of oil. They torqued the bolts so hard it deformed the gasket and caused the leak. It turned out every valve was way out of spec. So much so I was worried I was performing the work incorrect. The procedure is complex as you need to find top-dead-center for each cylinder. I put her back together using a new valve cover gasket, applied sealant at the required corners and torqued the bolts to specification. The leaks and rattling stopped. I had the car for 300k miles, and did all of my own maintenance work. Thing ran forever. One thing I learned over the years is the valves rarely need major adjusting. Maybe one or two would be slightly off. But never like I saw that first time. My guess is the first shop never adjusted the valves at all per the 15k mile maintenance schedule. The second shop, because I took it in explicitly for the adjustment, had to attempt the adjustment but did it wrong. I am really skeptical anybody's valves are being adjusted as part of the 15k service. It is so easy to skip and almost impossible to check.
As for brake fluid replacement. It is possible the mechanic will drain each wheel twice and torqued the bleeder valve to the exact correct tension. It is also possible they will use a turkey baster to suck the brake fluid out of the reservoir, fill so it looks new and moved to the next customer. You hope they have the integrity to do it right, but they are incentivized to do the latter.
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