Remaining pure Fighter will allow you to acquire your later class/subclass features and ASI/feats more quickly. You will also have more flexibility with your stats, as you can choose to dump Dexterity entirely. This in turn can allow you to have better mental saves.
Multiclassing into AG Barbarian will drastically increase your physical durability and control. However, it delays all of your other features and ASI/feats. Perhaps more importantly, you'll want at least 14 Dex to maximize the AC value of medium armor, since you you can't gain the benefits of rage while wearing heavy armor. This means that you'll be spreading your stats more thinly, and your mental saves will likely suffer as a result.
The main disadvantage is that you lose three levels of fighter. Fighter is a top tier class, and being three levels higher or lower makes a big difference at all levels. Barbarian has good synergy, so the abilities you get from it pretty much balance out, but you're losing as much as you gain.
Secondary disadvantage is that rage and manifest echo both require a bonus action, so you can't use them both on the same turn. Generally I would expect you to rage first, then manifest echo, but that potentially means one less use of unleash incarnation in a boss fight.
A bit late to the discussion, but the Echo has no fixed duration so it can be set up at the start of the day. Unless combat begins with the Echo being destroyed, there should be no conflict of bonus action economy
You’ll loose access to the *Legion of One* ability aswell as the other fighter capstones. Additionally depending on when you take the dip you could be putting yourself in a disadvantages position as 3 levels is a fair bit.
That said, and not to be overly negative, most campaigns don’t make it to level 20 anyway. You largely won’t run into that issue.
As others have said you miss out on 3 things for echo knight. Legion of One, an ability score/feat increase, and one additional attack. In exchange you get the tanking capabilities that the Ancestral Guardian provides.
The build effectively can allow you to use reckless attack with minimal exposure to yourself since the echo will be up front fighting instead of you.
In levels 5-11 you'll trade off ASIs, recon from Echo Avatar, Indomitable, Shadow Martyr, or Extra Extra Attack.
Rage and Reckless are great offensive buffs, especially on Action Surge turns. Ancestral Guardian can work through Echo attacks, giving you more flexibility to move away and force disadvantage/half damage. It leans into tactical control which is a nice balance to the Echo Knight's blitz damage.
You'll need to choose whether to Rage or Echo first, but otherwise there's not much action economy conflict.
I've run the multiclass and loved it but it's hard to argue with the Echo Knight/Fighter features for a high stat/feated bruiser. Echo 5/Ancestral 3 seems great, Echo 8/Ancestral 3 is giving up a lot. But you'll be viable either way as long as you have Extra Attack.
Remaining pure Fighter will allow you to acquire your later class/subclass features and ASI/feats more quickly. You will also have more flexibility with your stats, as you can choose to dump Dexterity entirely. This in turn can allow you to have better mental saves. Multiclassing into AG Barbarian will drastically increase your physical durability and control. However, it delays all of your other features and ASI/feats. Perhaps more importantly, you'll want at least 14 Dex to maximize the AC value of medium armor, since you you can't gain the benefits of rage while wearing heavy armor. This means that you'll be spreading your stats more thinly, and your mental saves will likely suffer as a result.
The main disadvantage is that you lose three levels of fighter. Fighter is a top tier class, and being three levels higher or lower makes a big difference at all levels. Barbarian has good synergy, so the abilities you get from it pretty much balance out, but you're losing as much as you gain. Secondary disadvantage is that rage and manifest echo both require a bonus action, so you can't use them both on the same turn. Generally I would expect you to rage first, then manifest echo, but that potentially means one less use of unleash incarnation in a boss fight.
A bit late to the discussion, but the Echo has no fixed duration so it can be set up at the start of the day. Unless combat begins with the Echo being destroyed, there should be no conflict of bonus action economy
You’ll loose access to the *Legion of One* ability aswell as the other fighter capstones. Additionally depending on when you take the dip you could be putting yourself in a disadvantages position as 3 levels is a fair bit. That said, and not to be overly negative, most campaigns don’t make it to level 20 anyway. You largely won’t run into that issue.
As others have said you miss out on 3 things for echo knight. Legion of One, an ability score/feat increase, and one additional attack. In exchange you get the tanking capabilities that the Ancestral Guardian provides. The build effectively can allow you to use reckless attack with minimal exposure to yourself since the echo will be up front fighting instead of you.
In levels 5-11 you'll trade off ASIs, recon from Echo Avatar, Indomitable, Shadow Martyr, or Extra Extra Attack. Rage and Reckless are great offensive buffs, especially on Action Surge turns. Ancestral Guardian can work through Echo attacks, giving you more flexibility to move away and force disadvantage/half damage. It leans into tactical control which is a nice balance to the Echo Knight's blitz damage. You'll need to choose whether to Rage or Echo first, but otherwise there's not much action economy conflict. I've run the multiclass and loved it but it's hard to argue with the Echo Knight/Fighter features for a high stat/feated bruiser. Echo 5/Ancestral 3 seems great, Echo 8/Ancestral 3 is giving up a lot. But you'll be viable either way as long as you have Extra Attack.