The lack of arrays, dicts and local variables when trying to be POSIX compliant becomes quickly annoying when writing big programs though. There are of course workarounds to deal with those but I wish we didn't have to use them.
What's interesting with this example of command1 | command2 is that some shells such as zsh will optimize the last member of the pipeline to be executed in the current process (nothing mandated by POSIX here), so effectively this works on zsh.
> [[ ]] is a bash builtin, and is more powerful than [ ] or test.
Agreed on the powerful bit, however [[ ]] is not a "builtin" (whereas [ and test are builtins in bash), it's a reserved word which is more similar to if and while.
That why [[ ]] can break some rules that builtins cannot, such as `[[ 1 = 1 && 2 = 2 ]]` (vs `[ 1 = 1 ] && [ 2 = 2]` or `[ 1 = 1 -a 2 = 2 ]`, -a being deprecated).
Builtins should be considered as common commands (like ls or xargs) since they cannot bypass some fundamental shell parsing rules (assignment builtins being an exception), the main advantages of being a builtin being speed (no fork needed) and access to the current shell process env (e.g. read being able to assign a variable in the current process).