If my information is correct, Rumble used to use connection-based matchmaking, but then switched to skill-based matchmaking at some point in D2's lifetime. This makes sense.
However, I can't seem to find a game where it would be working properly, unless my idea of a skill-based system is horribly wrong.
While I understand that one's Glory rank is not the singular factor in determining one's skill, I feel like it is a pretty good indicator of it. Now, I myself have never reached max Legend rank, with my highest Glory ever being about 4000. So, if skill-based matchmaking is working correctly, then how does it decide to match me with at least 1 person who is [i]Unbroken[/i] in basically all the Rumble games I play? I can't even exaggerate this. Maybe in 1 out of every 30 matches played I get a game without any [i]Unbroken[/i] people. I believe this started around Shadowkeep's release, and in general the Crucible has been less of a fun experience. I'm perfectly okay with getting second place, but not if second and first place has a 6+ point difference between them. I just don't see how this ends up being a fair match.
EDIT: Case in point, the 3 Unbroken people in my current Rumble game.
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As far as I can tell, it matches people on connection and skill. When a full group is found, they try to balance it out. Typically one good player on each, one poor player on each and then try to balance out the rest of the group. It's basically never 12 people that are close to your skill. Skill is not just your highest glory ever, or your win loss or kd ratio. It's a combo of many things that Bungie looks at.