1- Can PoA be used on Mainnet?
No, PoA cannot be used by Mainnet. Any public blockchain mains privacy by not disclosing which address(public key) associated with whom. No one needs to disclose identity for participating identity in public blockchain. In PoA authorities need to disclose their identities for participating in the blockchain. In public blockchain, all the participant can participate in the consensus process while in PoA only authorities will participate in the consensus process.
2- what are the demerits of using PoA on Mainnet?
PoA cannot be used on Mainnet.
3 -How efficient is PoA?
PoA can be compared with consensus algorithms of the private blockchain. One can not compare PoA with consensus algorithms of the public blockchain. According to an abstract of this paper:
The analysis advocates that PoA for permissioned blockchains, deployed
over the Internet with Byzantine nodes, do not provide adequate
consistency guarantees for scenarios where data integrity is
essential. We claim that PBFT can fit better such scenarios, despite a
limited loss in terms of performance