Since I started this thread I will show my understanding about most opinions I saw the last few days:
There are people who agree with OP and offer solutions:
1. "You can just change it from X to Y"
Yes, for now I am playing the game as designed, but once all core books are out it will maybe be a good time to start changing things (like my most hated healing rules), and I will use the solutions presented on this thread.
You all already have my thanks.
There are people who simply do not care:
2. "It's a level 20 ability, most people play at level 4~15"
Well, ok.
I intend to do it soon.
3. "It's a capstone ability"
Yes, one that is so superior to another classe's capstones that it's hard to design an encounter to challenge a moondruid and a monk/barbarian/fighter/rogue at the same time.
And others defend that this ability is just fine as it is, no need for any change:
4. "You can't shape from one form to another."
It's pretty clear that, by raw, you can. An animal is just as physically capable of turning into another animal as a human is physically capable of turning into an animal.
It's a class feature, if it transfers to your beast form, then you can use it.
Yet, it is a good houserule. And it will be good if designers change to that with an Errata/FAQ.
5. "A DM can just add more creatures"
If the moon archdruid have a chance of not surviving an encounter, no martial class will survive as well.
The DM just killed everyone because he tried to challenge the druid.
6. "A DM can just use the right creatures with save or die"
That is just my personal opinion, but a DM having to greatly narrow types of encounters down and putting monsters where they should not be just because of one classe's ability means this ability was not well designed.
7. "Even if you are immortal, that makes no difference if you are just standing there taking hits and doing nothing"
Moondruid resets HP as a bonus action, on his real action (standard action or whatever) he is still a capable 9th circle caster, or 8th circle if he takes my advice and cast foresight.
8. "It's just a PvP problem"
I was introduced to two examples of monsters, Tarrasque and Pit Fiend, that are suposed to be very strong, but both seem to have a great difficulty / zero chance to win against moondruid.
9. "It's just a PvE problem, an "other class 20" can win"
If you find a way to Barbarian 20 have even the slightest chance to kick moondruid's ass, please share, I want to try it.
Even fighter 20 with action surge hell is not enough, again, if you make all calculations and manage to kill his beast form and druid form in one round, I want to see it! (remember that if you assume the fighter to have magical weapons, then do so for human form druid's armor and shield too).
If it's another caster I understand that moondruid will maybe win, but most likely lose, no problems there.
Conclusion
10. Is the ArchMoonDruid absolutely invincible?
Most certainly not, he is vulnerable to some things, but is invincible to a lot of challenges that should pose a threat.
11. Is that a problem?
Not if you ignore 20th. If you are like me and intend to play at 20 by raw in the future, then it is a problem.
*sorry for english mistakes, it's not my native language