it is very easy to make an unplayable character in any skill-based system
I don't believe that is necessarily the case. I've helped players from steam forum who had lackluster builds. Beat encounters that they deemed were too "impossible" for them with their own character builds. I've rolled plenty of gimped builds through Faythe and my melee builds. For example, Faythe being restricted to only starting pistol. The point is to illustrate that a bad character build, or weapon, isn't going to make the game unplayable. First, in regards to builds, the learn as you do type system makes it easy to avoid some of these pitfalls you might fall into during character creation. This is because by doing easy encounters beforehand. You develop combat skills at a rate which makes up for any deficiencies you might have by misallocating stat points. So lets say you go low perception - many people complain about missing enemies in steam reviews. Well, if you do the first arena fight. Then thugs near apartment. Followed by Evan's encounter. Your weapon skill will increase, granting bonuses towards accuracy. If you pick up optical implant and Warrior feat. Accuracy will be at a decent level despite the misallocation of perception. In some instances you can get away from not picking implant or warrior. Just do enough encounters and weapon skill will make up the perception loss. Lastly, by getting consumables early, then using them against the enemy to level the odds towards your favor. To illustrate, irregardless of build, well placed flashbangs can win encounters alone.
Speaking of using consumables. I remember a while ago. A player who had a stock pile of gas nades, smokies, and a couple of stasis; but was somehow struggling with the Jonas siege. When he was kind enough to give me his savefile. I managed to beat it simply by using those resources to win. Funny enough he blamed it on character build, even though that wasn't the case. Poor fellow restarted with new build when he didn't need to.
It's not that difficult. The game, quite frankly, gives you enough tools to bypass a player making bad build.
I think players having problems with the game can be grouped into two groups: 1) Not using the resources the game provides to be successful; disruptors, stims, gas nades, smokies, flashbangs, implants, et cetera 2) Not doing side encounters to level up skills - it actually makes a difference using side encounters to get snowball of stats rolling.
Anyways. I think Hero Mode will be great addition for players who want to see everything on first run: for example there was a solo player in steam forums complaining he wanted to see everything in his solo runs while also being combat machine.