Proof of Stake coins have many issues here are some:

PoS is not backed by anything other than the belief there are worth anything and there will be an endless supply of PoS coins because one created today does not have a significant advantage over on created tomorrow, next week, next month.......
What currently is happening is new coins are created with PoW, mined for a week until a fixed number is reached and then change to PoS and then you can claim your stake at buying xyz coin. The only advantage a coin released today has over on made sometime in the future is; somebody already bought into it. The advantage quickly disappears if the new coin has a better catch phrase a flashier webpage or bigger marketing capital....
There is no end in sight for stake claimed coins and all promising x % return if you know a bit of programming you will have your very one coin too and everyone can buy into your claim based coin completely deluding the marked.
Its a barrel without bottom and once it clicks by the herd run for the hills if you own a stake.  
  
With a PoS the richer get richer. Nothing more to add to it that's just how it is.

To 51% PoS is dead easy:
You start aggressively buying until you have 51% of a PoS coin, and then sell off your coins so that you no longer have 51%, but your history of having once owned 51% makes it possible to attack the network at any time in the future at next to no cost only some computing resources (and thus electricity costs, etc.).
As you once had a 51% stake, you can build a better blockchain than the other 49% can, starting from the point where you owned 51%. You develop this blockchain in secret, after you have sold off your coins (and profiting from it); and then release your secret blockchain to the world, and nodes will pick it up because it carries more stake than the 49% blockchain.  Now not only do you have your profit from the original sales of the coin, you have your 51% back (to the extent that it's worth anything).  Not all coins need to be in one address, in fact, doing so would prevent the attack in most PoS implementations.

 
PoW is backed by energy. There is no better backing than energy because everyone needs it, wants it and i will never have any problem selling it. To create a PoW coin you need x amount of energy and you can not cheat. The best you can hope for is to have  a more efficient miner. Because the energy has been spent, the coin has a base value (many other things on top) and is a kind of a storage medium.