Well, I haven't seen the soils reports or seen any raw data from any core sampling or test pits, but I'd hazard a guess that most of the soil out there is of very poor quality, with the land under Contemporary and BLT likely being the best given that Contemporary was the only tower on property for years. You'll get no argument from me on that.
Having said that, and again not knowing the in-situ water table depths, soil composition, stratification, or bearing capacity, it's pretty likely that several of the sites could be developed with existing technology. Whether it's near-surface consolidation through import and fill, mud jacking/compaction grouting, cement deep soil mixing, or the advancement of piles down to competent bedrock, poorly consolidated, saturated, or heavily organic material can be made competent. Just look at some of the huge construction sites on reclaimed land in Japan, Hong Kong, and the Middle East. Heck, just about all of Tokyo
Disneyland sits on reclaimed land in Tokyo Bay. Granted, those are coastal reclamation and not on or adjacent to swamp land, but the technology has advanced since ground was broken on WDW, and continues to develop to this day. "Not suitable" can be become "not economical" then evolve into "not ideal" and then "feasible".
Certainly, it's not cheap, and if you've got good dirt elsewhere, that's where you obviously build first, but over time soil changes, and technology advances. Also, the type of construction is obviously dictated by soil conditions, and if Disney is on a tower building trend (as they seem to be with Gran Destino, Riviera, and DLT), then that can affect buildability and site selection, and that certainly could drive and/or limit site selection. Alternately, a resort more in the low-rise style of say SSR, OKW, or even PVB might be more feasible in a broader range of locations, with far less, yet still significant, site prep. I'd bet a paycheck that's why you keep hearing about "activity" occurring on some of those formerly "abandoned" sites: additional ongoing soil testing performed over time to monitor changing conditions and buildability based on current needs and technology.
I also wouldn't base any purchase decisions on it, but conversely I wouldn't assume that Disney isn't planning on swinging any more hammers in the next 20+ years either.