Seems to me right to add some clarifications to what was said here.

What actually those secondary stats do?

Initiative: nothing to add here to what icex said.

Control: if you and your enemy own 0 control stats, and this is what happens in arena, then you own a 5% chance to fail in controlling your enemy (so to apply to it blind, sleep, chaos, immobile, acupuncture effects).

Control secondary stat actually modify this basic chance by 1% every 100 points of difference between the stat owned by the ninja that deliver the controlling effect and the ninja that suffers it.

So if your ninja owns, lets say, 2000 control and the targeted ninja owns 8000 control, then your chance to fail in applying the negative effect is 5% + 60% = 65%.

So if you own 11000 more control than your enemy is sure the enemy can't control you? No, because this chance is capped at 95% (and, in the opposite case, at 5%).

Little supplement: if my ninja is immune to debuffs and doesn't deliver any controlling effect (example Utakata) is Control Stat for me useless?

No, because if a controlling effect is delivered by a CHASE and the enemy control fails on your ninja then your mystery DOES NOT GET INTERRUPTED if could be interrupted by it.

Combo: if you and your enemy own 0 combo stats, and this is what happens in arena, then, unless stated differently by the specific standard attack description, you own a 15% chance to apply the secondary effects of a standard attack.

What are the secondary effects of a standard attack? Every effect that is not the pure damage. Example for Gaara standard attack are to deliver High Float and to self shield. For Water Style - Water Whip are to deliver Knockdown and to apply Acupuncture.

Combo secondary stat actually modify this basic chance by 1% every 100 points of difference between the stat owned by the ninja that deliver the standard attack and the ninja that suffers it.

So if your ninja owns, lets say, 2000 combo and the targeted ninja owns 6000 combo, then your chance to apply the secondary effects of your standard attack is 15% + 40% = 55%.

So if you own enough combo secondary stat is sure the enemy cannot apply the secondary effects of his standard attack on you?

As for Control stat also here the chance is capped at 95% (and, in the opposite case at 5%).

Little supplement 1: if the difference between your combo stat and the enemy stat is higher than 100% then also limited to standard attacks evasion skills (like han's or hinata's) begin to fail.

Little supplement 2: if a ninja owns a standard attack that says: '... and have a chance to deliver x combo' (like for example Utakata standard attack) the difference in combo stat between the attacking ninja and the defending ninja also modifies the chance for the best outcome to happen. In Utakata case, for example, his standard attack can land or 6 or 8 or 10 combos. The higher your combo stat in comparison with the defending ninja is and the lower is the chance Utakata does land 6 combos.

Little supplement 3: when in the description of the standard attack is said: 'a fixed chance' then that basic chance is not 15% but 30%, when in the description is said: 'an high chance' then that basic chance is not 15% but 75% and in this specific case there's no cap at 95%.

Critical: if you and your enemy own 0 critical stats, and this is what happens in arena, then you own a 15% chance to land a critical hit on your enemy.

Critical secondary stat actually modify this basic chance by 1% every 100 points of difference between the stat owned by the ninja that deliver the attack/mystery and the ninja that suffers it.

So if your ninja owns, lets say, 10000 critical and the targeted ninja owns 6000 critical, then your chance to land a critical hit is 15% + 40% = 55%.

So if you own enough critical secondary stat is sure that every attack you land is a critical hit?

No, also in this case the chance is capped at 95% and at 5% in the opposite case.

Injury: nothing to add to what ICEx said except that the increase of the critical damage landed is always ROUNDED DOWN, so if the difference in injury stat between you and your enemy is 299 then you get +2% (and not +2,99%), if is 425 then you get +4% (and not +4,25%).

Ending note: what I said before applies in those terms ONLY if yours and the enemy level is the same.

A level difference between a ninja and another ninja actually widens the basic chances i talked about.

By how much per level is hard to say, but level indeed plays a role there (Ninja exams ring a bell there to you all, i'm sure).

So if you own a lower level than your enemy you need also to overcome the malus granted by the difference in level by adding secondary stats.

you could stack life recovery only if you are able to stack over 800 of it and only in a full healing setup, like the triple healing team with sailor sakura, azure fang and hokage tsunade + 1 tank (life recovery stat gets multiplied by healing tips).