I was tempted to use the much more intimidating 5e art but decided to go with the official picture in the end.
Regular gnolls were terrible, and to this day remain one of the rare monsters not just deemed too weak for their RHD, but too weak for any amount of RHD. Even orcs (regular orcs, not water orcs) had considerable advantages that made it hard to pick gnolls over them.
The flind gnoll is stronger than its regular cousin, if nothing else. The stat adjustments (+6 strength, +2 dexterity, and +4 constitution, no penalties) are pretty good.
...sadly, aside from that there's very little that justifies playing a flind gnoll. The +2 bonus on charisma checks to influence gnolls is so situational that it might as well not exist, darkvision is common, and +2 natural armor is hardly special either. The only thing that might be interesting is weapon familiarity with flindbars (fantasy nunchucks).
This weapon, rarely mentioned in optimization discussions, is an exotic 2d4 bludgeoning weapon with an expanded crit range (19-20), the flail's bonus to disarm attempts, and the ability to make free AoO-less disarm attempts on a crit. Note that disarms can also be used to make an opponent drop their spell component pouch, holy symbol, or handheld magic item, making it useful against casters too.
Consider the following numbers:
Spoiler: Some numbers
The following assumes starting scores of 16 strength, 14 dexterity, and 16 constitution, as well as 10 in all mental stats (so, 32 point buy). The flind gnoll has a single level in barbarian, the other races have three, making all of them ECL 3 characters (because it's obvious flinds don't deserve +1 LA). Damage calculations assume all characters are power attacking for max damage and use a greatsword. If the characters are using a one-handed weapon with a shield, or not power attacking, the flind's advantages only grow because power attack is a comparatively smaller part of its damage output. I didn't run any numbers on two-weapon fighting, but can only assume the flind has an edge there too.
Flind: 34 (8+4.5+6.5+3x5)
Human: 34 (12+6.5+6.5+3x3)
Orc: 34 (12+6.5+6.5+3x3)
Water orc: 37 (12+6.5+6.5+3x4)
Flind: Armor + 5 (+2 natural, +3 dex)
Human, orc, water orc: Armor +2 (+2 dex)
Flind: 14 (8+2+4)
Human: 30 (20+5+5)
Orc: 18 (12+3+3)
Water orc: 18 (12+3+3)
Flind: 20 (2d6+9+4) at +6 (2+6-2)
Human: 17 (2d6+4+6) at +3 (3+3-3)
Orc: 20 (2d6+7+6) at +5 (3+5-3)
Water orc: 20 (2d6+7+6) at +5 (3+5-3)
Alternatively, the flind can use a two-handed flindbar build which deals 2 less damage per hit, grants occasional free disarm attempts, and deals bludgeoning rather than slashing damage.
Furthermore, the flind gets longer rages (though the others will have more starting next level) and has better Fortitude saving throws.
As can be seen here, even when compared to something like a water orc the flind holds up relatively well. It lacks certain tasty racial feats (something something headlong rush) and has less skill points and HP, but hits more often and enjoys superior AC. The flindbar build sacrifices damage for utility against equipment-using foes and is not without merits either.
At higher levels, the flind's HP and skills begin to catch up with the water orc's, which hopefully compensates for the latter getting class features earlier and having an easier time entering PrCs. The human is obviously still a prime pick thanks to its free feat.
Considering all of this, I think the flind gnoll is balanced at +0 LA. It's playable and makes certain interesting playstyles slightly more viable.