Mouthfeel and tenderness are the result of collagen in the muscles being broken down into gelatin. This happens slowly at low temperatures and increases in speed as the temperature is increased.
A hard working muscle will have a high collagen content and will therefore require a longer cooking time.
Roasting cuts can be from hard working muscle groups like rump/round or low working muscle groups like tenderloin. You need to know where on the beastie your roasting cut resides.
The longer you cook, the more collagen is converted to gelatin and the more tender your meat will be. There is a sweet spot for everybody where the texture and mouthfeel are just perfect to their preference. Cooking beyond this point will simply result in more collagen to gelatin conversion and more tenderness.
The description of a piece of meat as mushy is a subjective appraisal. What seems 'mushy' to you might be fork cut perfection to someone else.
If you continue to cook the meat for long enough that all of the collagen is converted it will lose its structural integrity. Then everyone would call it mushy.
The thing to remember is everyone has an expectation of what texture they expect from a piece of meat. Yours will be different to mine. You will need to experiment to find your point of preference.