Is it always beneficial to claim PA if I and my spouse have rental or business income? What if we elected PA but turned out to be disadvantageous to us, can we revert that election?

Depending on income level and personal allowances, PA is a possible way to save tax. In case you have elected PA but it will result in more tax to pay, your income will be automatically assessed as if you do not elect PA.

Reference: https://www.ird.gov.hk/eng/faq/pa.htm