RJ45 ports by spec are magnetically coupled (Why Are Ethernet/RJ45 Sockets Magnetically Coupled?). However, one reason against the use of shielded twisted pairs is the possibility to create a group loop between equipments, as discussed in Shielded Twisted Pair (STP) termination and grounding.
The built-in isolation transformer should at least provide some degree of isolation for ground loop. Assuming there is a continuous connection from the shield to ground (patch bay, keystone, etc) Are the built-in isolation sufficient for this ground loop? Until what point will they no longer be able handle the group loop and an external one would be required?