I just started reading through the rules and the only hang-up I’ve had so far is with the Acrobatics proficiency. It seems a bit too open for abuse (that and I’ve always had a dislike for the tumbling proficiency from 2e - it smells of cheese). If I’m reading this right, I can have fighters with a 4 Dex and wearing full plate and shield do a flying backflip over someone. 
There needs to be some restrictions placed on this. Something like Acrobatics can only be used if you’re wearing leather or no armor. And either the pc’s dex mod should be applied to the roll or there should be a minimum dex mod for using it.
I don’t think Dex should factor in at all, but the armor bit makes sense.
Duskreign’s Minion here.
I agree that Dex should not be a factor. Limiting it to Leather makes sense. There is, however, a historical case for allowing it to work with Plate. Knights and squires were expected to be able to tumble in thier armor, both to encourage practice and to force them to seek a proper fit (remember Knights were very expensive). Also consider the functional difference between leather and plate armor; both are comprised of solid plates attached with straps, the only difference is the material and thus the weight. It is possible for someone strong enough to make acrobatics work in plate armor.
It’s a 5% chance for a bonus on your attack throw, with some minor positioning: I think that’s a marginal enough benefit to justify play-testing it. What abuse are you imagining?
I guess the major sticking point for me is the wording “somersault or back-flip behind an opponent”. I don’t care how much practice you have in armor, ain’t nobody doing back-flips wearing 40 pounds of it. And it’s not just a 5% chance, your chances goes up 5% with every level of experience.
Also, since there’s no penalty for failure, I’d expect players to try it every single round which will make me want to choke them. There should be some penalty for failing. Perhaps if you try and fail, you lose your attack this round.
Good catch. I concur that Acrobatics should be limited to wearing leather armor or less. It never occurred to me that it would be used otherwise, but of course in theory a plate-armored character could take the proficiency and do so.
In practice, Acrobatics is rarely taken by characters other than assassins, thieves, and nightblades, and they tend to be in light armor.
It is not a good or balanced idea to cause characters to lose their attack if they fail their check, as for most of the game the character will have a 50% or less chance of success. It requires you to use your move, which is enough of a requirement.
UPDATE: Characters with an encumbrance of 6 stones or more may not somersault. Note that elven nightblades automatically begin play with this ability as part of their class.
Making it based on encumbrance makes more sense than just armor.
makes sense although that will force us to actually use the encumbrance rules 
The encumbrance rules are so good, I want a reason to use them. 
Duskreign’s Minion here.
Encumbrance is a great way to manage it. I would like to point out a way to break this IF the intention is to not have plate armor wearing back flippers; magic armor.
Under Encumbrance
Armor and Shield - 1 stone per point of Armor Class*
- Magical armor and shields are lighter than mundane items. They reduce their encumbrance by 1 stone per point of magical bonus.
This allows for +3 plate (3 stone), 2 handed sword (1 stone), and a couple of odds and sods (1 stone) = 5 stone = back flips. On the bright side by the time you have +3 plate you should be fairly high level.
It’s +3 plate armor. It’s supposed to be lightweight. From my perspective, Darth Vader can do a backflip in his custom plate armor, and so can you.