From my understanding a quincunx can show up as stress, strain or circumstances being forced. A square is more about restrictions or challenges and feels like two things at cross purposes. An opposition is about polarity/duality that lacks harmony. Its like two things that are constantly challenging one another. It calls for compromise or balance.
Sounds a bit like they are the same but there are subtle differences.
For instance, I have a Mars square Sun aspect in my natal chart. I often just dont know when to stop working or putting my energy out that I can literally wear myself out and effectively deplete my Sun's ability to shine. (cross purposes)
I think if this were a quincunx instead it would probably not be so harsh. I think I could still tire myself out but it wouldnt go to the level of the square. There would be strain but not exhaustion. If it were an opposition I think it would likely show up that the Sun would exert its energy only to be twarted by Mars, and then visa versa. The two would see-saw back and forth until a balance is finally reached.
Maria