the topout spacer is most desirable, as you mentioned, BUT the most that can fit there was either 1" or 1.5" so above that, you have to have BOTH, such as my daystar kit, or a cornfed...
you can only get up to a 3" spacer lift, as opposed to a larger lift, but as far as disadvantages goes, the spacer lift does not give you a nice ride, it just keeps your ride how it is, maybe a TNY bit more stiff...
with real suspension lifts, you can get a nicer ride...I'm sure there are other things, people please chime in if ya know!