Let me try to explain:
The first example you mentioned was Attacker that heals while the skill is up, for this example i'll use Noel (a gold bishop that we got on a star trial event). Whenever you place a damaging unit you want that unit to do damage, but since every now and then you would want some healing over damage you would use the skill so she can heal in that moment and then she can return to damage
The other way (Healer that attacks while skill is up) is... imo harder to plan for. You see, whenever you plant a healer you want that healer to HEAL all the time, so even if you need the attack you would be sacrificing healing which may lead into a dead unit because you wanted to make your healer attack instead of fielding a temporary witch or some other damage dealer
The way i see it is: You put your healer because you know you want X unit healed if needed, so the option of converting to attack is not often taken, thus making Yuno a rather niche unit
Then again it's my opinion, you can go and read or watch videos and judge by yourself
Remember at the end of the day only can convince yourself![]()