Example of context-free tree language which can not be generated by monadic CFTG
Based on the comment of Michael Wehar, I've found this grammar to be that one which doesn't have an equivalent MCFTG:

$S\rightarrow T(a,a)$

$T(x_1,x_2)\rightarrow T(b(x_1,x_2),c(x_1,x_2))$

$T(x_1,x_2)\rightarrow a(x_1,x_2)$

This grammar produces perfect binary trees where in each branch the left child has a label a if it is a leaf, and has a label b otherwise; the right child has a label a if it is a leaf, and has a label c otherwise.

I don't have a very rigorous proof of this fact, however here below there are some observations.

It makes it impossible to build such a monodic context free tree grammar which will produce the same language of trees. This is because any single-ranked non-terminal can not properly set a parent node label (b or c). This label depends on "context": a knowledge about wether this node is left or wether it is right. 

Also, because the depth of each path in the given tree is the same, it can not be produced by a grammar where each next branch is generated by an independent non-terminal.