Since there is no way to bypass a POA, I guess very less people read the POA carefully and analyze it. Also, irrespective of how harmful the clauses can be, one of the most "harmful" clause is present in POA of every broker - because it is a SEBI guideline. Brokers are allowed to sell shares in your account esp to fulfil your obligations towards them.
One of the most common misuse is that they will trade in your account and if there is any profit earned, they will keep it for them, but if there is any loss incurred, they will debit it from your account. The only way to catch this abuse is to keel an eagle eye on your trade book and contract notes. Preserve the contract notes. The most common targets for such abuse are accounts with very low activity. So keep your account active and they will know that you are vigilant.