EDIT: Here is a list reported by Google search but I really do not understand or know how reasonable these challenges are.
Equal Protection: Owners may argue the tax unfairly treats similar properties differently based on second-home status, value threshold, or owner residence. This is likely a weaker challenge because tax classifications usually receive deferential rational-basis review.
Nonresident Discrimination: A challenge could claim the tax targets out-of-city, out-of-state, or foreign owners rather than property use. The law is safer if written as a tax on non-primary luxury residences, not on nonresidents as a class.
Assessment Inequality: Owners may challenge how the city values condos, co-ops, townhouses, and mixed-use properties. This could be significant if the $5 million threshold is applied inconsistently or without reliable valuation rules.
Due Process: Owners may argue the law lacks clear notice, proof, exemption, and appeal procedures. This would be especially relevant for disputes over whether a home is truly a second home.
Residency Conflicts: Taxpayers may challenge inconsistent treatment if the city treats them as NYC residents for income tax but non-primary homeowners for this tax. Clear coordination between residency rules would reduce this risk.
Home Rule Authority: Opponents may argue NYC lacks authority unless the state clearly authorizes the tax. This challenge is less likely to succeed if Albany passes valid enabling legislation and NYC follows required procedures.