power to weight is certainly probably the major factor - you need to be around 6W/kg to be a GC contender.
But teams would also be looking at changes in performance in altitude, and performances in a flatter TT as well where W/CdA matters. Most teams will plan out the race calendar, and if they have multiple GC contenders, encourage one to aim at the Giro, another at the TdF, etc.
But one major issue that isn’t really quantifiable is fatigue resistance and whether they can maintain performance over the full 3 weeks of the tour. Most teams have a “Plan B” with a rider who is potentially a stage-winner. If their GC guy is no longer in contention, they will target a stage where the terrain favors their B rider (whether this is an uphill sprint, a breakaway stage, etc)
Team Ineos have handled this one by nominating a couple of riders as “protected” in the first 2 weeks, and seeing who is the stronger in the 3rd week. You could see this in the way the handled the Woods/Thomas crash - Bernal kept going with the bunch, while the rest of the team stopped for Thomas.