This is a substantial paper. Here the authors provide a new package, ubms, easy-to-use, formula-based interface for fitting occupancy and N-mixture abundance models in a Bayesian framework using Stan. The package also provides tools for visualizing parameter effects, calculating residuals, and assessing goodness-of-fit. This package makes easy, faster and accessible for practitioners the use of some models including, e.g., random effects. I only have minor issues in Abstract:

Abstract
L 13-16. I think it would be appropriate to change the wording slightly. The objective is not to assess abundance while accounting for imperfect detection, but to make unbiased abundance estimates, for which it is necessary accounting for imperfect detection.
L 17-20. It is not clear how point 2 of the abstract relates to the content of the ubms functions described in Table 1 and in L 87-88. Perhaps it would be appropriate to add in point 2 of the abstract "...and other models"?

Supplementary materials
I’ve been using the grouse ubms application, and it works fine.
The manuscript is certainly very interesting, very well conducted and of general relevance.