It appears that the trouble starts with 3.0:1 and gets worse with higher ratios. This pertains to air shocks the most. You should also understand that the leverage ratios you, me and Danno have quoted pertain to average leverage ratios throughout the stroke. The actual leverage ratio varies as the linkage compresses.
Air shocks typically have a max PSI of 300, if you have to exceed that to get proper sag, you have to look for a different bike. You also increase the risk of failures associated with seals which are more prone to letting air pass when the pressures are increased. I'm talking about the dreaded stuck down issue with Fox air shocks.
As Danno pointed out, there is increased wear on the damping circuit and there is also a possibility of you requiring custom tuning if you can't get the shock's rebound slow enough or of you're overcoming the platform by sheer weight of your body.
All in all, however, do not despair and don't be too worried. Going with a 2.5:1 average leverage ratio bike, or there abouts, ought to be OK. You'd be better off with a coil shock, too.
_MK