Okay, here is one for you then, come up with a better methodology. Checking some instances stats 1.5 years after they have been released is simply not viable. Checking first - last boss shows the average difficulty of the instance.
If you check the average % of guilds wiping on normal mode bosses. You get these results after 1 month of progress.
from easiest to hardest (lower number = less % of guilds wiping on bosses = easier raid)
T9 10: 5%
T9 25: 7.38%
T10 10: 8.34%
T10 25: 8.67%