Are the extended storage alginates really more stable than traditional alginate impression materials?

Yes, some products are more stable. In a study recently conducted by researchers at THE DENTAL ADVISOR Biomaterials Research Center, three extended storage alginates [Alginmax (Major USA); Hydrogum5 (Zhermack); Kromopan 100 (Kromopan USA)] were compared to a traditional alginate material [Jeltrate Plus (DENTSPLY Caulk)]. After five days of storage in 100% relative humidity, the dimensional change of two extended storage alginates (Hydrogum5, Kromopan 100) was less than one percent.

Dimensional change of extended storage and traditional
alginates after five days in 100% relative humidity.