The 2% Rule is a repackaging of the Gross Rent Multiplier or GRM. The GRM is essentially another way of saying how many years it would take to pay off the asset with 100% of the gross rents.
The big things to look out for that cause the investment quality to drop are: 1. Lower quality neighborhood 2. Lower quality product 3. More deferred maintenance
Do not use the 2% rule as the end all be all for underwriting of an income property. It will leave you making questionable decisions.
Do not let it drive you to asset classes or neighborhoods where you are not comfortable owning.
You can use the 2% Rule as a screening criteria to weed out properties that do not match your model(s). You should use it to compare similar assets.