Ah, I think I understand now.
My attached image shows - from top to bottom reading downwards - (a) original text, (b) text cropped top and bottom, (c) text cropped by a shape. I assume you want to do something similar.
Scribus has no cropping functionality for text. Scribus always tries to render text as fully as it can, meaning that if it can't fully display a character it will not display it at all. One of the basic tenets of Scribus was to render text as well as possible. Cropping text does quite go against this.
Some cropping can be done on images but that's really just an artefact of having to put images into frames (except for vector art which is a different thing). Cropping sort of "comes for free" with image frames rather than it being a specific function.
I don't think it's as simple a case as the UI not exposing any cropping functionality available in Cairo. I rather suspect that it's much more complicated than that but I'm not interested in a discussion about the whys and wherefores. Scribus doesn't do it and the reason for that is up to the developers.
There are various ways to "crop" text in Scribus - depending on what effect you want - but the more complex techniques usually result in the text being non-editable. Sometimes you can just put a shape over the text, and you can even get the background to show through if you are careful. This keeps the text "intact". Sometimes, however, you have to convert the text into outlines and then manipulate those outlines in various ways. The original text is "lost". It really depends on the circumstances.
The Transform functions usually apply to frames/shapes and not the things inside the frames/shapes, except for rotation. I can see where this could be a bit counter-intuitive. I have no idea why there's a difference in functionality between some of the transforms in this context. It's just something that you need to learn and remember. Changing the functionality now could cause a lot of people a lot of problems.
One thing to remember is that Scribus is a DTP application and not an illustration application. It has a lot of illustration functions but it's never going to have them all, and it doesn't need to. As long as you can import things from other applications like GIMP, Inkscape and LaTeX then you can produce almost anything you want. You just might have to pop out to another application to get some of the more fancy stuff done.