You mean a worse version of the Marking rules DMG 271
It's main feature is a weaker DMG optional rule + a attack which is a ribbon you can barely perform.
No not at all. You need to read the rules. The Unwaivering Mark stacks with the DMG Mark if you play with that and they are two separate abilities.
Unwaivering Mark gives the opponent disadvantage on attacks against anyone except the Cavalier. It is unlimited use.
The DMG Mark on page 271 provides for advantage against an opponent if it causes an AOO, it is also unlimited use. This is on something anyone, including a Cavalier who used Unwaivering Mark can do.
If you play with that rule a Cavalier can do both of these at the same time. Further the Cavalier Hold the Line Ability makes the DMG Mark more powerful and vice versa.
If you are playing a Cavalier and use the DMG Mark on the enemy and he moves at all (not just leaving reach), Hold the Line Triggers an AOO, you make that AOO with advantage from the DMG Mark and it does not use your reaction as per the DMG Mark.
You have this completely backwards - playing with the DMG Mark makes a Cavalier much more attractive, allowing him to use one of his class abilities without using a reaction and the Cavalier can use the DMG Mark when other classes can't.
How?
It has CON mod uses. That's Max 5 per long rest. And you need 20 COn to do that.
Because they are attacking with disadvantage already if they attack that person, and they need to be within 5 feet. It also needs to be either a close hit or a high damage hit or it is not worth using your reaction.
Here is the string of what has to happen to use this:
Needs to be within 5 feet
You need to have your reaction
Needs to decide to attack ally, usually with disadvantage, instead of attacking you.
Needs to hit enemy (usually with disadvantage)
Needs to be significant enough to use your reaction.
That string of things does not happen a lot, especially since this comes online at 7th level and Hold The Line comes online at 10th level and you will be using your reaction on that a lot.
The core aspect of answering the question is how the martials deal with its challengers at all levels and how they display the fantasy at all levels..
Sure and 5E is easy enough thatr they have no problem dealing with challengers.
The only time it is really a challenge is levels 1 and 2.
The current power level afforded to the Cavalier, Samaurai and even Purple Dragon through the class and subclass abilities afford these fighters all they need to handle 5E challenges all the way to 20th level.
If fighters were outmatched by the foes they face it would be different, but they generally arent.
The Samurai has too few THP to brush of the increasing damage of monster. CR 5 monsters can wipe off a level 20 Samurai's THP in one attack.
No it can't. No monster doing any amount of damage can wipe a woth level Samarai because of the Samauria's 18th level ability that prevents him from even being wiped.
In any case not many (any) CR5 monsters could wipe out any 20th level fighter, but of those Samaurai would last longer than any.
The Cavalier lacks the number of Unwavering Mark bonus attacks and Warding Manevers to do each once per combat and is underdamage if they don't. And it deals less damage than the Champion as is.
In most adventuring days they will not have an opportunity to use all their WMs. I have played this class and this is just a false statement.
They can run out of the bonus action attacks, but I don't think most are going to have many rounds where they would be using that and can't. More importantly though they can use Unwaivering Mark every single turn and theoretically up to 9 times a turn.