Hum i quite disagree with that. She has her sleep which is her best defensive option and her grenade can save her life. I know that it depend a lot about the situation and skill’s management but i do think Ana is the best solo healer currently.
Not the most stable because you need to hit you targets but she do have a good healing burst (+block heal) and more than decent heal overall (80hps increased under nade effect). Plus with the buff to her ultimate, that’s another 300hp burst.