I actually think the much bigger problem is the amount of level inflated monsters from 4th Edition. Which conversely, might be making the others appear weaker.
Level Inflated monsters.
1. Dragons: Dragons should not be Epic Monsters. Tiamat is an Epic Dragon, Ancient Red Dragon should cap out at Level 20.
2. Balors & Pit Fiends: These are not epic monsters. Orcus and Dispater are Epic demons and devils. Balors and Pit Fiends should be statted in and around the mid-high Paragon Tier.
3. Yuan-Ti: In my opinion, no naturally occuring (medium sized) humanoid race should have its basic rank and file troops above Level 10.
Humanoids (from non-hostile terrain): Basic Troops Levels 1-5 (Humans, Orcs, Gnolls etc.)
Humanoids (from hostile terrain): Basic Troops Levels 6-10 (Drow, Yuan-ti, Kuo-Toa etc.)
Humanoids (from planar): Basic Troops Levels 11-15 (Githyanki, Rakshasa, Illithids etc.)