Background: The main factors related to regulatory decision outcomes reflect the pragmatic expectations of physicians to deliver safe patient care and uphold trust in the profession. Yet empirically understanding critical ones, like concern seriousness, has proven difficult. Methods: In this cross-sectional study, we developed a new quantitative measure of concern seriousness via expert focus groups. Using multivariate modelling, we investigated its statistical association with regulatory decisions about all physicians involved in the UK over the 10 years ending with March 2023, within a population of 416,610. Decisions by stage were: whether to investigate complaints; investigation outcomes, including adjudication hearing referral; and hearing outcomes. We controlled for dozens of potential confounders, simultaneously addressing many live questions; giving adjusted odds ratios for associations of outcomes to: criminal conviction, seriousness, allegation types, complaint sources, practice type (e.g. GP, paediatrician, trainee), workplace history, personal characteristics (e.g. age, sex), international/domestic qualification, socioeconomics (e.g. childhood deprivation), exam results (e.g. IELTS), and many more. Results: Severe outcomes predominantly related to factors similar across stages: criminal conviction, high seriousness, allegations of criminality, honesty or fairness, and the complaint coming from the physician's employer, police, or others acting in a professional capacity. Controlling for seriousness chiefly adjusted associations to conviction and allegation types, explaining some away. Conclusions: All else equal, the seriousness of concerns was one of the main factors related to regulatory outcomes. Such main factors underline what is practically expected of physicians to avoid for safe and trustworthy medical practice.