actually I do lots of embedded, busybox is about 2MB and bash is 1MB, unless the resource is extremely restricted I just install bash for scripting purposes.

python is about 4.5MB and Lua is about 200K, both run on top of a shell anyways, and need some packages to be fully useful.

To be fair adding 200KB Lua (no luarocks etc) on top of a shell is useful sometimes on embedded system, but I rarely need that.

In addition to bash itself being widespread, most of the common "bashisms" are also supported by zsh and most variants of ksh (and many systems actually use some variant of ksh disguised as /bin/sh). This is unsurprising given that bash itself was originally meant to be a clone of ksh. The places where they differ tend to cluster around uncommon/newer features (like coprocesses), certain syntactic quirks (but if you're aware of them they're usually easy to work around), and interactive features (which are mostly irrelevant for scripts). If you're writing simple scripts (or you're writing complex ones in a careful way), you can reasonably "target" bash and expect them to work on other common shells.

If you're going to be distributing shell scripts, it's probably a good idea to test them on a handful of common shells anyway. If portability between shells isn't something you're worried about, it's exactly as reasonable to ask users to install bash as it is to ask them to install mksh or zsh (or even python or lua or whatever) anyway. If you're most comfortable with bash, just use bash!

But for the love of god and all that is holy, please don't write csh scripts ;)