Interoperability between libraries that expect your code to be pure R / pure Python. If you use RCpp or Cython or CPython then you lose much of the magic behind the language that enables the cool (but frequently slow) features. My biggest pain point in this situation: you can not use SciPy or Pillow or Cython code with Jax/Pytorch/Tensorflow (except in very limited fashion).

Differential equation solvers that need to take a very custom function that works on fancy R/Python objects is another example of clumsiness in these drop-to-C-for-speed languages. It works and as a performance-nerd I enjoy writing such code, but it is clumsy.

That type interoperability is trivial in Julia.

I think the argument is that most R users don't know C++. So Julia avoids the "2 language problem" that you get with modern scientific computing.

Not much an argument at all, if you ask me. There's definitely a benefit to only having to learn a single language (rather than R and C++), but the library/package ecosystem in R is hard to beat; unless you're doing truly bespoke computational work, the number of mature statistical libraries/packages in R is unmatched. Rcpp's syntactic sugar means most slow R bottlenecks can be written in C++ almost verboten, but without the interpreted performance penalty. One of R's best and under-emphasized features is its straightforward foreign-function interface: it's easy to creating bindings to C/C++/Fortran routines (and Rust support is coming along as well).

I've been impressed with Julia, but it's hard to beat 25 years of package development.

Same argument as python.

In other words, you can (empirically) get a lot done that way, but there is always friction.

One thing I don't like about the two langauge approach - deployment story get's more complicated it seems?

In my case I went to deploy on a musl system and things with the two language just were a pain to get up and running.

Conversely, everything that was native python ran fine in a musl based python container.

Your native python code just moves also nicely between windows / linux / etc

I think that's just being clumped in with "Use C++," which he mentioned as an option