Why Angels can´t stay on Arboreus? -> we don´t know yet wether they can permanently stay on Arboreus or not
Why letting Humans permanently stay on Arboreus would make them too strong?
I guess I can type my thoughts again, but would be cool to hear the input of other people as well, since maybe I am thinking wrong 
Let´s start with the aspect of the Playstyles. Beastmen are for people who want to be able to play safe PvE, and maybe if they get sick of that try to get evil, but they have to go to Syndesia for that because according to Feature Spotlight #6, Beastmen are not allowed to choose any Alignment other than Good on Arboreus. The reasons for Arboreus being a safe place are:
-> The more evil a character is, the less time it can stay on Arboreus (Good Alignment - Neutral Elignment).
-> Characters with Evil Alignment get rejectet by the Planet, get a huge debuff, the normally neutral monsters start atting you and you can only stay for half an hour.
-> ~75% of Arboreus is a safe zone where people without Good Alignment can´t enter
Now, for a beastmen to get into PvP and getting "evil", takes a lot of effort, time and there is no going back. So for your character it is a one time ticket. Once you are an abomination... have fun entering Arboreus, because you get treated like a player with Alignment Evil.
So, as we can see, Beastmen are for PvE lovers. The most likely only PvP for them could be the rare occurance of an invader on the planet, or traveling to Syndesia for whatever reason.
Humans are meant for a PvX playstyle. While having PvE, PvP also is just as much a possbility. Syndesia will most likely be the hotspot of interracial conflict. The humans living there, most likely being in conflicts with themselves, the demons raiding them and beastmen trying to turn into abominations. While having PvE on the planet Syndesia, it is in no way safe like on Arboreus. The only possibly safe places are towns with guards, which are set to attack players with Alignment Evil when spotting them. So on Syndesia, every action you take, comes with a certain risk.
Additionally to that, there is also the Eclipse, in which demons are free to roam the lands of Syndesia. For the other races it is also easier to travel to Syndesia, then the other planets.
As for the racial bonuses of Humans, they get more Attribute points than the other races, making them more flexible in their playstyles ("classes"). On the topic of technology we don´t really have any information yet, how much influence this will have, but I think only Humans are able to use it, so other races buying it will probably not be a thing.
The only time such a technology is mentions is in the lore (or short story) of a town on Syndesia in the Feature Spotlight #1.
So, going with "Humans don´t really have a racial advantage, so why play them, when doing anything on Syndesia comes with a risk?". Humans are the race with the most diversity on how you can play them, you can go back and forth between being Good / Evil several times, which other races can´t do. Of course it will take some time, but the possibility exists. If another race wants to change their playstyle, they can never go back.
I am trying to find a good reasoning to why I think letting Humans permanently stay on Arboreus would be bad for the playstyles, but all I can really think of that a Human would gain from it would be a "Safe Zone". A Human player would then be able to freely choose if he wants to live on Syndesia or on Arborues (which becomes possible, when you allow them to stay on Arboreus an unlimited of time). They could then also join a beastmen guild. So if a Human player get´s into a big conflict, or just does not want to live with the risks of living on Syndesia anymore, he could go ahead and go live on Arborues. He could live like a Beastmen, while also having the advantage of the technology. And that´s where the problem lies... I don´t think it is good for the game, that a race can have the same playstyle as another race, since they "made" the races, to distinct between different playstyles.
Another thing I want to touch on is the topic of ressources. The three planets will have different rare materials, so Syndesia will most likely also become the place for interracial trade (in case this even will be a thing), since traveling to Syndesia for a Beastmen / Demon is far easier, then traveling to Tartaros / Arboreus. Now, a Human with a Good Alignment is able to stay a decent amount of time on Arboreus, so they have some access to the rare materials of Arboreus, but it still comes with a risk. Giving them free access to Arboreus just gives them free access to the materials of two planates, which another race does not have, and thus giving them an advantage in this area as well.
I like to see it as, anything you do with a Human does have / has to have a certain risk, because Humans have it the easiest to access both other Planets and have "free" access to all playstyles. The risk I am talking about is of course, most of the time, getting killed by another player, regardless of race. If you want to play without the risk of PvP, play a Beastmen. If you are okay with some risk, play a Human and if you only want to PvP anyways, play a Demon.
When giving Humans the possbility of permanently staying on Arboreus, you take away the uniqueness of the Beastmen as the safe PvE playstle.
Another thing this does is... if you allow a race to permanently stay on two planets, they then also have two Houses (one on each Planet) which gives them two bases of operation. Don´t really think that would be that big of a deal, just something to think about as well 
Why I think that for Angels it would not pose a problem to be able to stay on Arboreus forever?
Well we don´t know yet wether they can or not, but not everyone can just "become" an Angel. It takes a lof of time and hard work. I don´t mind that a few people who invest a lot of time and hard work possibly get an advantage over other players.
