Having used Tcl extensively I think scoping rules aren't really too complicated, but there are some apparent inconsistencies. A main one is global variables aren't visible inside procedures unless declared as "global", whereas procedures are global no matter where created (in the global namespace). However referring to variables and procedures using fully qualified namespaces is always correct and unambiguous.

The thing with upvar (and uplevel) is that by default upvar refers to variables in the caller of the current proc. But when a proc is called indirectly (e.g., a callback proc) then the variable upvar is intended to reference may be 2 or more frames above. Using something like "upvar #2 refvar var" works as expected. It's analogous to deferencing pointers in C when there are >1 levels of indirection.

Is there any programming language that doesn't have some confusing rules? If a language is going to be useful the answer is bound to be "no". It's a curious how we get used to a particular (or even peculiar) syntax. When used long enough it begins to feel "natural" and obvious, and find it amazing how anyone would have difficulty understanding how it works.