For those of you who watch my runs of this game, I tend to use Nelson over Flak. So, you may be wondering why I use Nelson. To explain this I need to go over Flak.
Flak tends to be faster but less powerful than Nelson, but he does have several things that keep him from being used.
1). Chain attack issues- When you do chain attacks with Flak, throwing one punch makes him do three punches, though the game only registers probably the first one as hittable, this also happens on the second hit of the chain. In between these first two attacks, it's possible for enemies to break your chain by attacking Flak. In fact, Flak needs angry mode more the Nelson does to make his attack more efficient.
2). Too many OTG (on the ground) grab setups- A good deal of Flak's attacks cause OTG setups; though it's good to use after a run attack, Flak's backdrop, leg sweep, and knee attacks aren't very useful especially when you want the damage of the backdrop and leg sweep, which are Flak's best attacks. The knee is just not very useful, unlike Nelson's low blow.
3). Terrible neutral grab attack- Like I said, the knee attack is useless as it'll cause a knockdown rather than stunning enemies and continue chain attacks.
4). Running slide attack can knock enemies out of bounds- While the slide useful for certain enemies and bosses if you're not careful you might knock them out of bounds and you have to wait till they reappear.
In the end, Nelson just has better options despite limited punch and run attack range.
1). The low blow can be used to stun lock enemies into a nearly endless chain attack.
2). Chain attack is consistent.
3). The run attack is the best option for OTG grab setups. Enemies that were thrown by powerbomb or hit by splash damage via frankensteiner can be OTG grabbed.
4). Frankensteiner has faster throw animation than the powerbomb and it doesn't require you to grab enemies from behind.