Imagine that every unit has a stat pool of like 1500 points. They have like 30 stats (random number) and can put their point into any stat. They can put 1500 point in hp. Or 50 in each stat. or 500 in "damage" and 500 in armor and 500 in HP or anything inbetween. Basically, if they choose more mitigation, they lose another stat they could have had instead (either offensive or defensive). SO yes, the tradeoff is meaningfull and yes it defines what playstyles you are going to have.

Careful with this, because having point-pool itemization doesn't, by itself, create meaningful choice. All you've described are choices that have consequences, but it's still possible for all of the consequences to effectively be the same. Balancing the itemization is a good last step, but you first need to sufficiently differentiate your stats. Give the user a clear reason why choosing a piece of mitigation gear will make them play differently than choosing a piece of health gear.

An example of giving the user a choice that isn't meaningful is armor vs health in Hearthstone. Some cards heal and some give armor, but (outside of a few special interactions) they're effectively the same thing. It doesn't matter how much they balance them, there's no orthogonality added to the play space if the only decision is between the two.

Imagine that every unit has a stat pool of like 1500 points. They have like 30 stats (random number) and can put their point into any stat. They can put 1500 point in hp. Or 50 in each stat. or 500 in "damage" and 500 in armor and 500 in HP or anything inbetween. Basically, if they choose more mitigation, they lose another stat they could have had instead (either offensive or defensive). SO yes, the tradeoff is meaningfull and yes it defines what playstyles you are going to have.

Careful with this, because having point-pool itemization doesn't, by itself, create meaningful choice. All you've described are choices that have consequences, but it's still possible for all of the consequences to effectively be the same. Balancing the itemization is a good last step, but you first need to sufficiently differentiate your stats. Give the user a clear reason why choosing a piece of mitigation gear will make them play differently than choosing a piece of health gear.

An example of giving the user a choice that isn't meaningful is armor vs health in Hearthstone. Some cards heal and some give armor, but (outside of a few special interactions) they're effectively the same thing. It doesn't matter how much they balance them, there's no orthogonality added to the play space if the only decision is between the two.

You are correct, I need my stats to be very diverse and have unique perks / drawbacks in order for the system to create real choices. It can be difficult at times since in the end most stats end-up beeing a way to deal more damage or to survive longer. But so far I've got a fair share of stats that I think might be all be appealing, depending off course on the strategy adopted by the player and by what he wants to be strong against.

My issue with healing is different tought since it is not a stat that gives passive healing but a skill that requires the unit to perform an action that has a time/cost associated with it. I'll however will have issues balancing healing-related stats such as life leech and life regeneration per second...

Small note about Hearthstone, i haven't played a lot but there is still the difference that HP can be healed and armor cannot. This may not be enought to create an interesting concept but the 2 stats aren't exactly alike :)

Someone probably already mentioned this linear solution but why not have your damage mitigation mitigate healing at the same rate? However if healing is a big part of the game then a damage mitigation role would be the same as a HP role. Perhaps make your damage mitigation class not healable in a regular way or have a limit on the amount of heals. Don't worry about the lore, you can always think of something to make the mechanic fit into the lore of the game.

Healing is a tricky topic for sure, as you want to make sure it's fair to the players while not being so OP that battles aren't an issue. If nothing else, just trial and error it until things work out to your acceptance.

I have a few ideas for you. (I'll admit when it comes to this type of thing I prefer brute force)

Healing Mitigation seems like it would fix your problem pretty easily while allowing you to come up with more varied interesting design. This isn't an uncommon concept either, the most common example I can think of is Grievous Wounds in League of Legends. It is a status that reduces healing by a percentage for however long the ability that applied it dictates. 

How you do it can depend on a lot of things, but it should be either item based (if there are any) or skill based. 
examples: if you want to do it on a numbers basis it could be something like reduces all healing by 1, or 1*the targets level, or something of that nature. 
so if you have a level 10 enemy with 100 health

then they are attacked, reducing their HP to 80 (as an example) and that attack applied the level-based anti-heal

instead of that enemy immediately healing for 20, thus negating your damage, they instead heal for 10, since their level is subtracted from all heals on that target.

There are a bunch of ways to go about this, for example if your game has a party system, you could make it so that a skill or weapon could cause its target to be a reduced SOURCE of healing.
you could have it so that an armor with high damage mitigation could have the drawback of "you don't heal as easily" to prevent finagling.

The reason why I'd recommend something like this is that it allows for a much greater amount of freedom with both playing and designing your game. The player can be a busted healer and it won't feel "wrong" because there's potentially things the enemy can do about it happening. It lets the player really get into whatever style of play they prefer. Also from a logical standpoint, if there are healing things in this RPG world where assassins or hexers exist, of COURSE there would be things to prevent healing, right?

but these are just my thoughts on the matter. My design philosophy is that everything should have some kind of viable counter-play.