Lv1 FA causes 15 frames of blockstun. When you dash out of FA, your dash is one frame faster than normal.
Chun’s forward dash is 15 frames. The dash comes out one frame faster, so she is at +1 after Lv1 FA, forward dash.
Most characters do not dash that quickly and will be at disadvantage after Lv1 FA, forward dash.
You can find info on forward/back dash speeds here: Super Street Fighter IV - Shoryuken Wiki
Lv2 FA causes 21 frames of blockstun. Only the slowest characters will still be at disadvantage with Lv2 FA.
As for the second question, the attacker is usually at advantage. How much advantage they have depends on the move being canceled… in some cases an FADC can actually leave you at disadvantage.
Frame advantage = blockstun caused by that move - forward dash frames + 1