Each service has its equivalent to AR 635-40, section 5-4(e)(1)(a) and 2, which reads as follows:
Section 5-4(e) Ability to deploy. The PEB considers whether the Soldier (Active Army and RC) is deployable individually or as part of a unit, with or without prior notification, to any location specified by the Army. (1) The provisions in this paragraph concerning the ability to deploy do not apply to the following DES cases: (a) General and MC officers. (b) When the presumption of fitness rule applies. (2) With the exception of the cases identified in paragraphs 5–4e(1)(a) and 5–4e(1)(b), the PEB will find Soldiers unfit who are medically disqualified for worldwide deployment in a field or austere environment.
As someone who regularly represents physicians in this setting, I can attest to the fact that it is quite a bit harder to have a physician found unfit unless they have significant psychiatric issues that impair their ability to provide competent care to patients. Even then, the PEB members expect the person to be enrolled in an impaired provider program.
Have I succeeded in having physicians found unfit and medically retired for physical conditions rather than psychological diagnoses? Yes, but it often means going through the whole process twice or more for Navy and Army physicians. In Air Force settings, it can take several RILOs over 12-24 months to have AFPC direct a full medical board and even enter the IDES process, let alone be found unfit.
The bottom line is that being unable to deploy will not guarantee that a physician will be found unfit. It typically requires their specialty consultant- think of it as the program manager for that particular medical specialty, like rheumatology, oncology, orthopedic surgery, etc. - to play an active role supporting an unfit finding by drafting a letter or testifying to the fact that the consultant is unable to find any billet in which the provider can function within their specialty. Additionally, you will need supporting evidence showing that they cannot perform any general medical officer functions.