Cross Server Queues to get more healers / tanks
Cross server queues will only increase the sample size for the group finder. Healers and especially tanks are usually hard to find.
For one the tank and the healer usually gets the blame when things go wrong. As tank your group can work with you or against you, but it's the group that decides which way things go.
If you have a group finder queue with 10% tanks, 20% healers and 70% dps on average on a single server, a cross server queue will not change that. instead of 5 tanks, 10 healers and 35 dps you will have ten times of then but the lack stays the same.
You need to encourage people to tank / by being nice to them, not to pull aggro and please leave CC's alone.
On PTS i have had groups that worked perfect, that would mark and even leave unmarked CC's alone / other groups will simply AOE anything or just dot a mark.
The only doable way to mitigate this is dual (or more) specs.