Extremely? There is a known tank and healer shortage. If you are a DPS there is ZERO guarantee that a tank or healer will pick your group over others. You can increase the likelihood of attracting a tank or healer by listing your group as being 1337 or something, but even then there's still no way to control another player and guarantee a run. And god help you if you're a dps class that the community currently views as sub-optimal.
Am I saying guarantee enough? Do you want me to post the definition?
You can't sit here and try to tell us that it's not a problem. It's one of the distinct advantages the matchmaking system has over LFG. And if a tank or healer leaves your group it gets you another, often quickly.
All LFG really does is act as a manual LFD system with only slightly more control over group composition, at the cost of the inability to do anything other than stare at the tool. You're still playing with randoms. You're still extremely unlikely to ever see the same group again.
This idea that matchmaking automatically leads to bad groups and nerfed content is wrong. The idea that forming your own group solves all problems and automatically fosters a better community is likewise false.