Dual agency means one agent, or one brokerage, represents both the buyer and the seller in the same sale. So your agent would owe a duty of loyalty to you and to her seller at the same time, on a deal where you want a low price and the seller wants a high one. That's the whole reason it's regulated so heavily, and a handful of states prohibit it outright. Which rules apply to you depends entirely on the state the property is in, so ask her to name the statute or the license rule she's operating under and get the answer in writing.
Both things you read come from different places. It isn't banned everywhere, and in states where it's legal it does get used on simple transactions. The "she knows the seller's bottom line" idea is the part to be careful with. If she's a true dual agent she generally can't tell you the seller's lowest acceptable price, and she can't tell the seller your highest, because that would be disclosing one client's confidential information to help the other. So the supposed benefit tends to evaporate at the exact moment you'd want it.
On commission, the agent may end up collecting both sides, which is where the criticism comes from. Nothing in the arrangement automatically reduces what you pay. The written compensation agreement you sign with your agent is the document that decides that, and it's separate from the dual agency form, so read them as two different things.