http://www.ssbwiki.com/Perfect_shield
Here are the basics.
Projectiles:
- No shield stun
- No shield pushback
- Reflects projectile (deals half damage)
Physical Attacks:
- Shield stun is unchanged (a common misconception is that you have less or no shield stun when you PS physical attacks; this is not true)
- Shield pushback is maxed out (equivalent to the lightest light shield; holding Z is the only way to get that light shield)
- When you release the shield button, you normally have 15 frames of shield release lag (I may be off by 1-2 frames). If you PS, the game will allow you to do A-attacks during this shield release lag. Thus, if you PS an attack, you can let go of shield and fsmash, dtilt, or do other ground moves more quickly than if you had to WD in place or undergo shield release lag before attacking.
Other stuff:
- You can only PS with a hard shield (the trigger must click).
- The window for PSing is shorter for projectiles than for physical attacks (I think it's 2 and 4 frames respectively).
- You don't take shield damage from PSed attacks.
To answer your original question, yes, there are times when PSing can mess you up. The most common instance (and only non-niche one I can come up with atm) is being pushed back too far to shield grab or counterattack.