Overhead starters?

Hi, so I have some questions regarding SF 5’s characters overheads.

  1. Are they all theoretically reactable to?
  2. Are all of them punishable/unsafe on block?
    3 Are any of them combo starters? For example in mkx almost every character has a low starter and an overhead starter which pretty much makes the neutral game and especially the wake up game severely 50/50 based.
  1. yes
  2. yes except a few of them (Chun, Ibuki, Karin’s rekka overhead; Alex is relatively safe too)
  3. Ken, Ibuki, Balrog, Laura in VT, Chun in VT, Necalli (charged VT overhead) and Urien (charged version) can all combo off their overheads