karazhan and zul aman were really well tuned as 10, they ofc became facerolly, but doing them in blues and later t4/t5 was reasonably challenging at the time, with no prior knowledge. especially trying for the mount in lower gear or during the first few raids there.
10s are great but we're talking about the difference between logistical difficulty and mechanical difficulty, getting 10 ppl together is easy. I think some ppl prefer the larger sized raids, I liked 40s it felt like there was more cumulative effort involved. but trying to get 40 ppl together and show up on the same days is where the challenge really is in classics raiding. some ppl like that masochism maybe, although these days it is difficult to find folks willing to commit to it for a long time.