We are developing a shiny application for microscopy image analysis. The point is that most image processing libraries are in python while many statistical packages to analyse objects spatially are in R and python.
We are trying to combine them not out of choice but to use the simplest packages for the job. Python is very easy in most cases to incorporate into R with the reticulate package. I found that the other way round to work with R in python is not as comfortable. If I have to use a class in both languages I make a python class.
The comment that we often get is why we are not just using python instead of python and R. I don't really have a good answer for this because it could probably work. But then shiny is very comfortable to use and there are packages in R that have been around a long time and seem to make the job easier for us.
Think about the core of your project and check where you get more community support and packages and go with that.