Complex sentence: The mirrors' advantage of simple machine geometry is maintained in machines which produce compact toroids, but there are potential disadvantages for stability in not having a central conductor and there is generally less possibility to control (and thereby optimize) the magnetic geometry.
Split into simpler sentences: The mirrors' advantage of simple machine geometry is maintained in machine producing compact toroids. There are potential disadvantages for stability by not having a contral conductor, and there is generally less possibility to control and therefore optimize the magnetic geometry.