I have completed 7 terms in UCMAS. Due to my shifting to new place i was unable to complete the graduation in it. So I guess I can answer it due to my experience. Pros we all know that increases your speed and efficiency in calculation. It does improves the concentration. But some of its cons are actually for younger kids. Especially of smaller classes as they just complete their calculations through tricks used in UCMAS. So they use same things in their school level papers and just solve questions without solving it by proper methods and this affects their grades in school.
I completed UCMAS 8 levels in 5th grade. I did not go for grand level as i thought to go for abacus. I have a lot experience in UCMAS. Yes it builds your thinking period, Strategy Development, Time Management and most important Mentally solving skills. I do not say that the one who does not do UCMAS does not aquire this skill. But the one who does has more accuracy and confidence . This are some pros mentioned above. This is really really great if you complete levels when you are kid say (2–5) grade. 1 grade student can to join it but he should know 1 to 10 tables .This sounds funny but I heard that time when mam told to some parents.
I had completed UCMAS that makes me feel an era old. However, for statistics i may tell you there had been 10 levels and 2 grand levels which were all completed by me. The number of levels might have changed with time.