As one of the chief complainers, I probably need to come to RM's defense at least a little bit with respect to its user interface. In many respects, the RM user interface is pretty good, and there is a lot of software that I have used that has a worse user interface. Also, the complainers do not always agree on how they would like the user interface to be improved.. For example, many new RM users these days seem to complain about the lack of an "all in one view" similar to view of a family you can get when browsing trees on ancestry.com. In this view, you can see parents, spouses, siblings, and children all in one view, and the view is very graphic so that you can navigate just by grabbing the box for an individual and dragging it around and the rest of the view follows. But I'm an outlier who hates that view in ancestry.com and who would not wish RM to expend valuable programming resources supporting the same view in RM.
Having said that, most of the complaints about the RM user interface revolve around it being so modal. That means that any time you need to do anything in RM it seems to open a sub-window and you can't do anything else until you dismiss that sub-window. And the first sub-window can require a second sub-window which can require a third sub-window etc. creating long sub-window chains that all have to be dismissed before you can do anything else. It is often the case that what you really need to do in RM would greatly benefit by having multiple sub-windows open at the same time. This would require that at least some of the sub-windows in RM be non-modal.
Another way to describe the same problem with the RM user interface is that many pieces of software have what are called wizards or sometimes that have a full blown wizard mode for the whole program. These wizards or this wizard mode leads you through the steps of doing a process one little step at a time. Such wizards can require an enormous number of clicks, each of which accomplishes only one little thing, making it very time consuming to accomplish a simple task. It's as if RM is always in wizard mode and there is no "expert mode" or "classic mode" that avoids all the clickiness.
And yet another way to describe the same problem with the RM user interface is that the user interface makes it extremely slow to do bulk data entry. For example, suppose you were entering a family of ten children. To enter this data into a spreadsheet with Microsoft Excel, you could create a spreadsheet with a column for last name, a column for first name, a column for birth date,and a column for birth place. You could very quickly and easily enter all the required data for all ten children all on one screen. There are many places in RM that could benefit from such bulk data entry. But instead, you have to enter each child separately and each piece of data separately. The problem is much magnified if (as you should) you include source citations and media files as evidence at the same time you are entering the rest of the data for the family. Indeed it is the entry of source citations and their supporting evidence that results in the longest sub-window chains of modal windows, and the RM user interface makes it extremely difficult even to see that the media files that are the evidence for the source citations.