I attended public school in the US, and wore uniforms for some of the years I was in school (but mostly didn't wear them).
My middle school started off with a relatively strict dress code when I was in 6th grade, and decided to institute uniforms when I was in 7th grade. The uniform consisted polo shirts purchased from the school (with logos, and in the oh-so-lovely school colors of bright red, bright purple, white, or black), khaki bottoms that were supposed to be specific brands (although that rule wasn't ever really enforced), and sweatshirts purchased from the school (that also had the logo, and were either red or black).
In theory, I didn't necessarily think the uniforms were a terrible idea. They relaxed some parts of the dress code when they introduced the uniform, so that was a plus. I didn't have any particular objection to wearing the same clothing as everyone else, and it certainly made getting dressed easier in the morning.
What I objected to was that the polo shirts and sweatshirts that had to be purchased from the school were cheap in quality, and more expensive than they ought to have been. As a result, I only had like 3 shirts and had to do a ridiculous amount of laundry. I also objected to khaki bottoms specifically. I started getting my period when I was in 7th grade, and as is common with lots of girls just starting, had no ability to predict when I would get it. I had multiple situations myself (and witnessed several more) where I ended up with stained pants or shorts because my period surprised me. It was made worse by the fact that I was lucky to have time to use the bathroom once in the average school day. I once had to beg a teacher to give me a bathroom pass (during a class called "enrichment", which translated to "do your homework or amuse yourself or whatever") so I could take care of a period emergency after the inside legs of my shorts were already massively stained. Given a choice, I'd have been in jeans or at least dark pants to avoid some of the embarrassment.
I also had very few clothing options outside of school, and when I started high school where there was no uniform. I'm sure my parents saved some money on my clothing while I was wearing a uniform, because I wore the same set of clothing for two years. But they spent all that they saved and then some once I was back to no uniforms and realized I had nothing to wear. Had I been able to spread out my clothing purchases over the two year period, I probably would have bought less (because I would have liked what I bought more). Instead, I bought a ton of new clothes when I started high school, but ended up not liking a lot of it because I was going for quantity and didn't have the luxury of waiting around for clothes I really liked to be available in stores.
I just really hate that I spent so much of my adolescence feeling like I looked awful. The colors available in my middle school's uniforms looked awful on me, and my khaki shorts/pants were both unflattering and (by the time I was in 8th grade) too short. Outside of school, I had very little to choose from. It took me most (possibly all) of high school to really settle into clothing styles that actually looked good on me. Teenagers have to deal with enough self-image problems already, and I really think that my brief stint with uniforms made mine worse.
The whole thing might have been better if I'd been in uniforms for my whole school career, or at least continued wearing them once I started. But having experienced a strict dress code/uniforms in middle school, and a pretty lax dress code in high school, I preferred high school. People wore inappropriate clothing at both schools, but I was a heck of a lot more comfortable (physically and with my self-image) when I got to pick out what to wear each morning.