Centurion:

He just gets to dish out way too much punishment for landing just one punch/kick/stunlock attack.
Simply put, the unblockable, undodgeable, unparryable, uncounterable never-ending chain of stamina-drain stunlocks and knockdowns needs to be fixed. Centurion grabs you once and you might as well put your controller down and walk away because you will not regain control of your character until you're dead, he never runs out of stamina and can just keep spamming the chains all day.

It might be okay if you play assassin with those super agile dodges, but as a heavy character, his kicks and punches cannot often be sidestepped, the only way you can dodge is to roll backwards.

Now most characters have an unblockable shove or headbut type move that allows a free hit or two if it connected and that's fine, that's how things should be to prevent players from just blocking literally everything. The issue is centurion can chain them before the target player has recovered stance and his follow up punishments are extremely damaging.

There either needs to be some significant delay between these moves to allow a dodge out or counter attack after being punished or they need to use enough stamina that you can't just chain them (maybe nerf the amount of stamina they drain from their target as well, to be more inline with other shoves/bashes in the game, because as-is one kick or punch drains over 1/2 my stamina).
It would also be nice if the tracking and/or aoe (whichever applies most) of these would be reduced enough so that the slower heavy heros stand any chance of dodging.
Plus, that thing where 1/2 his attacks stick into you and stop you from moving or doing anything is really annoying.

Shinobi:

The shinobi is definitely a lot more balanced than the centurion.
He hits like a truck and has some annoying range, but his extremely small health pool makes up for that.

The only real issue is the side kick is very easy to spam and very hard to avoid or punish.
Again, as a heavy, the side kick he has is borderline undodgeable unless you roll backwards. It also cannot be interrupted by a hit like most characters bash/shove attacks.
If by some miracle you do avoid it, the shinobi can recover and escape before any response attacks finish coming in.

It would be nice if the aoe and/or tracking of this was reduced so that it could actually be avoided or if it could be interrupted by a well timed attack.
Also, the ranged grab is a bit annoying, getting grabbed by someone that is off-screen and not showing on radar is a bit surprising, but I'd be happy enough if they just changed the side-kick move.

Generally:

I play on Xbox, so maybe this is just because you can change block stance and react faster on pc or something, but I have not seen this so-called "turtle meta". As far as I am aware, all the original characters have atleast 2 of the following:
-An unblockable shove/bash/charge move
-The ability to use well-timed grabs
-Unblockable attacks and/or unblockable abilities
and anyone using that character well can easily defeat someone who is only defending, especially if a teammate decides to help. Either way, introducing one character to counter an entire "meta" isn't the smartest move if you are at all concerned about gamebalance. It's like going, "Here, have a counter to everything if you spend money on the season pass or a steel pack". But in general it seems a lot of this complaining about "turtle meta" is just people crying because spamming basic attacks doesn't automatically win a 2v1, because, as someone who is really good with warlord, I can tell you that any 2 skilled players with original characters actually using their unblockables, shoves and grabs will tear you apart, even if you are good at parrying, even if you get revenge mode, even if you do the all-directional block. "Turtle meta", well, we're not all assassins, some of our dodges don't even get us out of the aoe of certain attacks, certain characters need to block, counter and parry to work at all as a character. If we're not supposed to block, parry, or use our counters, might as well take all the heavies and vanguards out of the game then.

Besides, assassins being able to avoid literally every attack is just as game-slowing and annoying as "turtling". If anything, playing a dodge oriented character would be easier because at least blocking and parrying has a directional and extremely sensitive timing component, assassins can just always dodge backwards if they are unsure.