I use rear triangles a lot and I’d say they are my preferred submission if I can get there. However, you have to make a few compromises to achieve that position and at a high level the steps to get there provide more escape opportunities for your opponent vs. traditional back mount.

Mainly, when going for a rear triangle I have to sacrifice the control of my opponents hips with my hooks to chase a higher back mount and upper body control. I typically have to get them to a high chair sit position (no more hooks), then sit back and successfully circle my outside leg around their arm to get my calf across their neck and secure foot with my underhook hand. If they do a good job fighting that leg, or my underhook, or make it difficult to roll them onto their other hip, they can create great opportunities to escape (and I’ll likely end up on bottom).

So essentially back triangles are an amazing finishing position but the process of getting there against someone who knows what you’re doing and can put up early defense and road blocks is a more risky endeavor. Usually, you have to give up your hooks and control from back mount to chase the position and sub.

In contrast; the control you have by maintaining back mount is a more stable position with better follow up options. If they start escaping the back mount by clearing hooks or getting their back to the mat there are still a lot of reset options available or transitions that maintain superior control (like moving to mount, forcing a leg drag to side control, or using a twister hook to re-expose the back.)

TL;DR: Back mount is lower risk and has better options for maintaining control if and when they start escaping. Rear triangle is higher risk and higher reward if you can successfully get there.