R quirks ... the <- syntax, the ~ formula syntax, looping through a dataframe is horrifically slow when you can't vectorize, multiple legacy object semantics, poor parallel/multiprocessing support, poor support for datasets that don't fit in memory ... upside is any statistical method probably has a decent implementation in R
python - most comprehensive ecosystem and libraries beyond statistics, e.g. Web, numerical and scientific computing, machine learning (Tensorflow), NLP, generally a good language to learn to program in, pretty easy and forgiving while also being reasonably expressive, performant, offering functional as well as object oriented features/styles.
the good folks at plotly are working on a shiny equivalent (dash) but it's not out yet. Django + matplotlib or bokeh or some client-side graphics like plotly.js is potentially powerful but not really as integrated as shiny.