Judging from the ranking of mechanical majors, the mechanical majors of the four schools are not bad. From strong to weak: Southeast University, Central South University, Jilin University, Shandong University. (Ranked in 2009)
Comparatively speaking, these three schools are local, not national universities like Tsinghua and HKUST. Jilin University is in the northeast, Shandong University is ok, Southeast University is famous in the Yangtze River Delta, and Central South University is second only to Huake in Hunan and Hubei. As for which one to choose, it depends on your future development area.
As for the exam, I haven't experienced it in school, but with the strength of these four schools, none of them is easy.
If I had a choice, I would choose Southeast University. Since you choose to take the exam, why not choose a better one. What's more, as far as I know, the graduate students majoring in machinery in Southeast University have high salaries after graduation.
I am an undergraduate of Southeast University, and I just studied literature.
Finally, good luck.