Beonin would never have been able to go back to the Tower if she had taken an Oath on the rod... the fact that she had even enough wriggle room to avoid her oath of fealty to Egwene speaks volumes to me about the difference.

Meidani was far more hedged in and if she had been given a direct order, say to jump up and down for 10 minutes, she would have had no choice but to comply. Beonin could refuse such an order if she believed that it was not something should be required to do.

In some ways I applaud Egwene for being so machiavelian that she caught these women in their own plotting and was able to forge at least a semi-effective tool out of them. I also think the BA hunters did what needed to be done to root out the BA. But the question I have is why leave the 4th Oath on Meidani and the other moles when they've clearly proven that they are not BA and would be easy enough to simply blackmail into obedience.