No medical school is a sure bet for acceptance. Every year some competitive applicants are not accepted at any medical school. No acceptance to a medical school, if on an Ed Delay from Army, they are accessed in another branch (MS, QM, AG, MI....).
USUHS publishes their class composition annually - I found these two quickly Class of 2017
and Class of 2019
My opinion is that overall it is more difficult to get into USUHS than to a civilian med school. It's a result of small numbers and that it is an Allopathic (MD) school. There are roughly 65 Army, 50 AF, 50 Navy, and 2 PHS slots at USUHS each year. Some of the Army, Navy/Marine, and AF slots now go to EMDP2 (Enlisted to Medical Program) applicants further lowering the number of slots for applicants off the street - It's a great program for enlisted applicants on AD and competitive.
Like any medical school though USUHS has a stated mission - theirs is the only one that includes military medical practice and supports military readiness. Most ROTC or Academy graduates align with the mission.
An acceptance to an Osteopathic (DO) school is significantly less difficult, but not easy, as their average numbers (MCAT, cGPA, sGPA, EC's) for acceptance are lower than most MD schools.
An acceptance to any medical school (MD or DO) is not straight numbers (MCAT, cGPA, sGPA, EC's). Apply broadly to both MD and DO, interview well, align with their mission, and hope for the best.
The AD service commitment from ROTC and USUHS is long. With no delays in education and the shortest medical residency (3 years), applicants are finished with their commitment at age 40. That's tough considering the first contract is signed at age 18.