I think the pain makes sense since several times VoAT is used, it's to forcefully transmit message to others. It's like shoving something in, the receiver also most likely isn't ready, so it pains.
Also, the pain part can ALSO be attributed to difference in strength, aside from proficiency and users' readyness.
- Sea King to Roger doesn't affect him. Sea King to FI Luffy bothers him.
- Luffy to child Momo hurts.
- Zou to child Momo hurts. Zou to adult Momo isn't.
Also, the pain part can ALSO be attributed to difference in strength, aside from proficiency and users' readyness.
- Sea King to Roger doesn't affect him. Sea King to FI Luffy bothers him.
- Luffy to child Momo hurts.
- Zou to child Momo hurts. Zou to adult Momo isn't.