0- The most important thing to understand is the backdash makes you in airborn state after the first frame, so on the first frame you're still on the ground to be hitted by any meaty or whatever
1- The backdash has 0 (ZERO) frames of invincibility
2- If a special hits you in the air depending on the properties of this special you can be juggle/combo after that
3- If a command move hits you, you can be in a juggle state in the air depending on the properties of this special you can be juggle/combo after that
4- After a normal move you're in recovering state, so you can be only hit by free juggle moves
5- Anti air throws will catch you
And if someone will poke you out of you're dp he will probably use a fast light in meaty and you lose or a j.CD and you lose again.
If the meaty is good made enough the dp will whiff or be guarded.
On this topic though..
Who out of all the characters in the game have invincible DPs from the first frame?
Roughly every A/B dp move has invincibility on his start up. I don't remind any exception. Some have invincibility after the first few frames following the activation.