It's nearly impossible to have any challenge with any 1 on 1 targets. Once you get a decent set of enemies on you at once, ones with decent stats, then it finally starts to get to be a little more exciting.
It would be cool if, like Sleeping Dogs, you could press the block button and it would block for like 3 seconds and if it didn't actually block any hits you get penalized and cannot attack for a few more seconds. In Sleeping Dogs it was counter attacking but... blocking can be the Bully version of it I guess.
Actually the one on one target system you stated is true and false at the same time. Although fighting one on one enemies are easy the more blocks and health they have the harder it gets. Russell is pretty hard to fight in free roam even with Jimmy's full move set so that is all not true.
The blocking system actually didn't make it easier or harder, everyone blocks manually even the player. The thing that makes the game easier is the evasive counter used by many AI and player's which could be a glitch or some faulty game mechanic. It's when you attack an enemy and they block but when you stop blocking and your arms open up you get attacked.
You can watch AI fights and view this happen, when Russell fights with the bullies style the recovery system disables attacks and blocks then allows for someone to get hit with a counter. Hard to explain like a greaser 3 hit combo, a preppy blocks and drops their guard then gets attacked with a greasers full move set instead of countering.
That is why the boxing style differs, it has an automatic counter even when blocking if you try to fight boxers with full block,evade, and attack abilities they will fight different from the other AI. I will upload a video showing what I mean, it is the movement Jimmy does when blocking that messes up an AI's ability to fight one on one if you notice it. One of the most messed up game-play mechanics is to move Jimmy while he is blocking they should disable your ability to move when blocking like AI's but I guess this is some sort of thing AI's were to have also but they never added it.