Like in most other card games on the playroom, pressing E in UNO will give you the number of cards in both your own hand as well as your opponents'.
Since the point of the game however is to be the first to dispose of your cards, and a common rule in UNO is to in fact diclair UNO, meaning 1, when you have 1 card left, I find this feature to be somewhat contradictory to the entire idea around which the game is played.
I propose a change in this functionality, only in regards to the game UNO, whereby pressing E will give you the number of cards in your own hand, but will not tell you how many cards are in the hands of those playing against you.
In stead, it would inform you that you simply cannot see the hands of other players at the table, after it shows you how many are in your own hand.
The return text would therefore display something like the following:
After a player presses E:
"Player: x (where x is the amount of cards in hand)
Sorry, you cannot see the hand of other players at this table."
While I understand that many players favour the fast play option in which the game proceeds too quickly for anyone to make use of this feature, I would point out that some players still prefer the traditional version and pace in which case the feature eliminates half the challenge, and is entirely against the spirit of UNO.
I believe that this would not only make the game more fair and true to its purpose, but will also restore the traditional challenge of following closely the progress of your opponent as well as your own, keeping track of their plays and draws until you believe they indeed have UNO.

You should keep in mind that when you play uno in reality you still can see how much cards someone has got. Still I understand that for some this might be more interesting and adding an option for this in slow uno, without interceptions could be cool but, this feature in uno with interceptions would be kinda pointless since you can't follow number of cards played even one VS one.

I've discussed this with a friend even as I was writing the post and they brought up the same point. In truth, the only way your opponent can see your cards is if you hold them fanned out as you would in most other card games. In UNO, this is wrong. When playing UNO cards are meant to be hold in a vertical line with the top card being at the very back, the rest of the cards hidden by the same palm in which you hold your deck. This throws off your opponent so he or she never quite knows when in fact you do have one card left.

As far as I know, the way you keep your cards in your hand doesn't depend on the game played.
Personally when I play any card game on a real table, uno, jass, spades or any other one, I always keep my cards in a single pile (or multiple piles sorted by color) in front of me, because I'm not eased to keep them fanned out. For other people it's certainly the opposite.

If I remove the information from E key, how do you know when you can catch your opponent for not saying uno then ? Even in 1 vs 1 game, you usually don't count cards.
And in the reality, you may not know the exact number of cards of your opponents, but you can still see if they have only one, or if they are dangerously approaching uno.

Agreed with Aminiel. Not to mention that, right now, there isn't any penalty for pressing u or 1 when nobody has uno, so any player can just wait about 2 seconds and then press it whenever their opponent has played in the hopes of catching them.

In complete agreement with Aminiel, leave this feature where it is.