Here's the thing; there is correct math and there is fun math.
Succeeding 60% or 50% of the time is fine initially, i can see that working for low level play and have no issues with it however as you increase in levels and experience, you should feel like your progressing and becoming better at the things your ment to excell at based upon your choices of where you want to improve so if those odds don't increase by about 10-20% respectively, your not really feeling like you've progressed between level 1 and level 10(+) which will make players feel like their choices had no impact while at the same time the system punishes players who opt not to choose the mechanically best options when they pick up such things for fluff reasons or because they appeared like fun to them.
People want to be competent at their roles as having repeated rounds of whiffs and therefor not having contributed at all does not add drama; it just creates frustration. I'd say let them be competent at their specialties as there are many forms of challenges in roleplaying games and not everything challenging should be combat per se... so why challenge them at EVERYTHING instead of the 60-70% they'll be bad at?
I played through the first 5 chapters of doomsday dawn and ran it as a game master, besides chapter 4 which actually felt rather enjoyable because it was more freeform and as such is what i enjoy in roleplay personally... i can't say i've had all too much fun despite the system having some interesting new ideas and things that are quite fun to utilise. Initially; my characters were massively flavorful and took thematic options but these days to avoid being dead weight to the players i'm playing with, i've been optimizing a lot more... not because i want to but because i feel forced to do so.
After chapter 5; the group i ran the playtest for had enough of the system all together and basicly gave up on it and truth be told i don't blame them considering most characters just feel incapable/incompetent unless dealing with lower level adversity and that chapter was ludacrous as far as playtest goals go for people who aren't getting paid for it.
Another thing i'd like to add is that; when people are talking about 60% or 50% chance to succeed that is for a first action; meaning your odds at succeeding on a 2nd are abysmal and on a 3rd action are practically non existant. The 3 action economy system was ment to add flexibility but the 50% benchmark math seems to interact poorly with it?
I just wonder how a long standing campaign is ment to be played, monsters hit frequently and their damage numbers are exceptionally high as most PC's can't tank a critical+normal hit and still be standing. The monster on the other hand is often still standing after PC's return the favor there. And the domino effect is a factor here as your "balanced" encounter just became heavely favored despite worse action economy because monsters get good/cool stuff this time around.