B+3xxRoll is something I've seen, but I don't ever see B+3xxExRoll, which is perplexing.
First of all, since MK has a lot of stuff that isn't generally hit-confirmable, B+3 to Roll is extremely unsafe if blocked. The EX will still get punished if blocked, but I've played with a lot of great high level players and I've never seen anyone block b+3xxEXRoll. The B+3 is a low, so if they get hit by it, the EXRoll is going to combo in regardless; however, if they block the low, the EXRoll is a quick Medium so unless they have the foresight and precise timing to stand up in just a few frames, they're getting hit. Realistically, it just doesn't often happen, especially if you don't use it every single time you approach.
It's just so unlikely it'll get blocked because there are only a handful of frames between when the low hits and when it's canceled into the quick Medium. Also, the setup in itself is a 50/50, because Mileena can just go low twice with b+3, b+4. Another nice side-effect from abusing this is the fact that it gets people focused on her low game, which allows you to throw in more u+4's. You can even set up for it by doing b+3, b+4 just once. Once they realize she has a double-sweep of sorts, people often think twice about standing. Also, in my experience, a lot of people just don't know that b+3xxEXRoll even exists, so it catches a lot of people off guard. It's not something that many people seem to know or abuse with this character.
One note though, it must be done close to the opponent. If you're hitting someone with the tip of her foot, the Roll will not hit them when she pops up. It has to be pretty meaty, but it's not hard to get that close to someone for a pressure string.
This is one of my favorite combo starters to bust out when people aren't expecting it, and it has served me quite well so far. In my experience, it's often spending a bar for just about a guaranteed combo.