Does being honest and open online have it’s downfalls? Is it better to hide behind a facade?

I think it's positive when a person learns that it doesn't have to be a matter of "pick and choose." One can be honest and open while at the same time having such resilience that anyone's (knowingly or not) trying to abuse it becomes not only harmless but possibly even constructive. Such resilience may be hard to come by, yes, but how else to acquire it than by being--what I also believe can be easier with distant people online than with acquaintances--honest and open?

(its downfalls)

Private chats only. Nobody can hurt you here and they are not censored.

it might

I learned the hard way that there are a handful of people that can make life pretty uncomfortable online should your friendship with them go south.  I think it's really wise to not share too much, too soon with people you don't know.

Trust must be earned.

Your avatar and username mean that no one knows your true identity.
That's a huge protection that can create a lot of freedom to just be yourself exactly as you choose.

Yes, there are risks in being open and honest, but I think the benefits far outweigh the dangers.
If something goes wrong one can always set a boundary or avoid that person in future.
These social sites do attract trolls, fishers, users, evangelists... people with agendas of all kinds.
Just Asking and the team are pretty good at responding to complaints and weeding out the trouble makers, so we get a pretty good level of safety.

If someone uses "you" language in making a judgement, I believe it's most helpful to not take it personally, but rather to think of it as a reflection of their values and thoughts.
I think a diagnosis like ADHD should only come from a qualified professional after careful tests and long term evaluation: if anyone else says it, it's likely to be pop-psyche garbage with no validity.
A big part of intimacy in any kind of friendship depends on honesty and openness; it's impossible to know or love without it. And that is a realistic risk we take in the process of opening up. We may discover unexpected things in the other that are no-go's or unacceptable to us. The the rejection or the need to part ways might hurt. Or the two might decide to accept differences and work around them.

My Dad used to say, "If someone criticises you, be objective and evaluate it. If it's not true, dismiss it. If it is true, decide if it's something you want to change, find the means, and follow through."

Only you can know who you really are.