Cons
1. Roof leaking water - but if you have good management, you don't worry much as all cost are borne by management. This is better than ordinary floor that if upstairs leaking , then you need to see the "face" of upstair owner, and wait them to fix.

2. Water pressure - Highest and 2nd or 3rd highest floor normally won't use water directly from water tank, but instead being served by a booster pump. In fact, the booster pump can deliver better water pressure than ordinary water flow from tank. But if you have poor management, that do not fix or maintain the pump properly, without the booster pump, the water pressure will be very weak.

3. Lift - Lift waiting time definitely cost sometimes, everyday up to a min or 2 is unavoidable, especially during peak hour of morning and evening, worst if one of lift broken down. This also depended on density of property.
Low density, the waiting time may be negligible.

Actually it largely depends on the management, if you have good management and always act responsively and maintain the apartment/condo promptly, those cons are not an issue at all.

It depends on density and how high the property is.

It is seldom to see a condo or apartment that not very tall or low to medium density being served by 6 lifts.

Most condo/apartment won't have more than 30, and with medium density layout, 3-4 lifts generally are enough.

It should be adequate unless you are having more than 12-15 unit per floor.

As said, a good management is the key, you can have 10 lifts but not properly maintained and 2-3 working, then how many lifts also no use.