A person to whom everything is freely given can never understand the value of those things.

Hardship can make a person worse. I'm sure we all know people who have become bitter because of the pain life has handed to them, and usually I find myself unable to truly blame those people. Hardship can break people.

But I also find that people who have never truly had to work [for something - not "job" work, all work], first do not understand the work that others have to do, and do not appreciate what they have. I recently read an update to a thread in r/relationships about a woman whose daughter was getting married for the third time and was pitching a fit because the parents didn't want to pay for this third wedding, as they'd paid for the first two and their following divorces. That daughter was handed money her entire life to subsidize her existence and it clearly had made her insufferable, entitled, etc. Some hardship is necessary, or we'd all be entitled assholes who think we just deserve everything to be handed to us.

I think hardship helps us to learn who we truly are as people. You don't know who you are until push comes to shove. I mean, that's what a lot of sci-fi and disaster fiction is about, right? How awful situations can either warp people into monsters, or people stick to their guns and their ideals. Hell, isn't that basically what The Walking Dead is about? (TV series, anyway, haven't read the comic)

At minimum hardship forces us to learn and examine what we truly value. "Is it more important to feed myself for six months, or use the food I've found to feed myself and three others for two months?" I truly believe we cannot know how we will react in these situations until they occur. We can attempt to predict our behavior based on our knowledge of ourselves but until we get to that point it's all a theoretical exercise, made from the comfort of a reality where currently, we are clothed, loved, and fed. (Presumably.)

We also cannot truly sympathize with people who are suffering unless we have suffered. Suffering gives us empathy. It gives us understanding and depth.

Sounds to me like Nietzsche suffered a lot and was trying to look for the upside, either that or he was a masochist.

I don't think anyone should ever seek out suffering and this quote has the danger of implying that because suffering can have positive effects, it should be sought out and relished. Yet at the same time, we should not flee from discomfort. If you use "personal comfort" as your compass to get through life you are not going to be challenged, and you are not going to grow. This parallels a conversation klein and I had about loneliness a while back. Don't hold up awful things as "noble." Don't cling to them because you think they make you better or special, a special suffering snowflake lilypad.

Let's flip the premise of your title and ask instead:

How can great personal growth be achieved outside of experiencing some type of hardship?

Is "challenge" a hardship? It could be taken that way.

No one needs profound self-contempt. When it is truly profound and all-encompassing I believe it prevents growth. Self-mistrust though - we should doubt ourselves. Not so much that it binds us from acting, but enough so that we see the room in our thoughts for other interpretations, reactions, etc. Enough so that we are willing to listen to others.

I prefer:

Per aspera, ad astra - through adversity, to the stars.