I can quote some of the relevant sections here (not supposed to share the whole thing). These are just some of the problems listed with the metrics there's quite a lot more but this comment is getting too long

The relative protein content, IAA content, and IAA profile of a given food are required to calculate the DIAAS. The FAO has not prescribed a specific methodology to determine protein content for the DIAAS but acknowledged that nitrogen con- tent can be used to estimate protein content for the PDCAAS [24]. Food-specific nitrogen-to-protein conversion factors have been determined for various foods and can be used for this calculation; however, the FAO does not recommend their use. Instead, it recommends that the generalized nitrogen-to- protein conversion factor be utilized [29]. The generalized factor was set at 6.25 because all proteins were originally estimated to contain 16% nitrogen; however, this varies great- ly between proteins [32]. Importantly, estimating protein content using the general- ized or food-specific factors influences the corresponding PDCAAS and DIAAS. For example, the food-specific factors for almonds and soybeans are 5.20 and 5.61, respectively. As a result, using the generalized factor to calculate their DIAAS yields 16.8% and 10.2% lower values, respectively, than when they are calculated based on their food-specific factors. Conversely, the food-specific factors for skim milk and yogurt are 6.36 and 6.40, respectively [33]. Accordingly, their DIAAS are higher when generalized factors are used. In ad- dition, greater discrepancies exist between conversion factors for plant foods than animal foods, with recent values ranging from 5.3 to 5.8 for grains compared with 5.85–6.15 for milk products [34]. The particular methodology used to calculate protein content therefore influences the DIAAS of plant and animal foods differently, decreasing scores for plant-based sources of protein while increasing scores for animal-based sources of protein. Due to differences in the ranges of food- specific factors, use of the generalized factor may also lead to more inaccurate scores for plant foods than animal foods.

Most literature examining dietary protein consumption and postprandial muscle protein synthesis (MPS) has focused on isolated protein sources, as used in the DIASS method, with limited literature focusing on the influence of whole foods on MPS [31•]. In most settings, protein is not consumed in isolation. Rather, whole foods are consumed with their intrinsic nutrients exhibiting a synergistic and concerted effect [48] and can influence the post-exercise MPS [49–51].

Raw foodstuff is used for most DIAAS modeling, whereas protein-rich plant foods (legumes, grains, etc.) typically un- dergo heat treatment, processing, or both before human con- sumption. Common cooking techniques modify proteins, with heat-treated plant-based proteins demonstrating higher digest- ibility compared with unprocessed sources [30, 52]. One such modification relates to the protease inhibitor trypsin, and pro- cessing treatments have been shown to deactivate as much as 80% of its inhibitory activity in raw flour [52]. Malting and fermentation processes can also increase the digestibility of some proteins, likely by bacterial protein pre-digestion and the lessening of “anti-nutrients” like oxalates, tannins, and phytic acid [31•, 53]. Their effects are significant, both for foods and supplements. The fermentation of grain coupled with other cooking techniques, as is often employed in tradi- tional cooking methods (e.g., sourdough bread), can increase the digestibility of grain protein to a level approaching that of meat [53]. Further, compared with untreated pea seeds, pea protein concentrate demonstrated 12% higher digestibility, matching the protein digestibility of casein [52].