I could not find another thread addressing this, so if it's a duplicate, I apologize in advance! So I like the UA variant class features a lot. Looking over them however I couldn't help but notice something specifically with Ranger. Having an alternative to Favored Enemy is great for customizable options, but considering Favored Foe replaces it, it doesn't seem to synergize with the Ranger's 20th level feature.

Favored Foe text from the UA:

Favored Foe:

1st-level ranger feature (replaces Favored Enemy)
You can call on your bond with nature to mark a creature as your favored enemy for a time: you know the hunter’s mark spell, and Wisdom is your spellcasting ability for it. You can use it a certain number of times without expending a spell slot and without requiring concentration—a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest. When you gain the Spellcasting feature at 2nd level, hunter’s mark doesn’t count against the number of ranger spells you know.

Then we have the capstone:

Foe Slayer

At 20th level, you become an unparalleled hunter of your enemies. Once on each of your turns, you can add your Wisdom modifier to the attack roll or the damage roll of an attack you make against one of your favored enemies. You can choose to use this feature before or after the roll, but before any effects of the roll are applied.

So essentially, by the way it looks as RAW, Foe Slayer doesn't work without Favored Enemy. The capstone seems a bit lackluster as a level 20 feature as is and now potentially, it's useless? Of course I can hear people already saying 'oh just multiclass instead' but not every game allows for that. You could leave it to DM discretion but that's always a gamble. Mechanically, if this is the case, it seems like there might have been an error made somewhere in class mechanic synergy? Does Foe Slayer work without the Favored Enemy feature? Going off of the flavour text rather than feature rules, does the Foe Slayer bonus then apply to those affected by Hunter's Mark from Favored Foe?

I haven't seen any errata address this, so yeah, RAW, the 20th-level ability would be useless with the UA ability. That said, I'd talk to your DM before you play the class with the UA rules. An argument could be made that the capstone ability could affect any creature targeted with your Favored Foe ability. I don't think that breaks the game mechanic any, even if it IS a bit redundant. 

The features work together just fine. The text of Favored Foe explicitly states: "mark a creature as your favored enemy."

Quote from SagaTympana >>

The features work together just fine. The text of Favored Foe explicitly states: "mark a creature as your favored enemy."

 This^^

Favored Foe

1st-level ranger feature (replaces Favored Enemy)
You can call on your bond with nature to mark a creature as your favored enemy for a time: you know the hunter’s mark spell, and Wisdom is your spellcasting ability for it. You can use it a certain number of times without expending a spell slot and without requiring concentration— a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest.
When you gain the Spellcasting feature at 2nd level, hunter’s mark doesn’t count against the number of ranger spells you know.

I agree, the replacement of Favored Enemy changes your Favored Enemy from a species to an individual.   The limitation is that you must now take a free action to  mark someone, but you get more to use against them in combat.

Quote from Mog_Dracov >>

I agree, the replacement of Favored Enemy changes your Favored Enemy from a species to an individual.   The limitation is that you must now take a free action to  mark someone, but you get more to use against them in combat.

 Favored foe still uses your bonus action to cast Hunter's mark.

I would also argue that casting Hunter's mark with a slot instead of the feature does not make them count as a favored enemy.

Quote from DxJxC >>

Quote from Mog_Dracov >>

I agree, the replacement of Favored Enemy changes your Favored Enemy from a species to an individual.   The limitation is that you must now take a free action to  mark someone, but you get more to use against them in combat.

 Favored foe still uses your bonus action to cast Hunter's mark.

I would also argue that casting Hunter's mark with a slot instead of the feature does not make them count as a favored enemy.

 Why would you argue that? One would think the fact that the same feature specifically includes the “Favored Enemy,” the free uses, and the spell as an automatic take would wrap them all in together.

Quote from DxJxC >>

Quote from Mog_Dracov >>

I agree, the replacement of Favored Enemy changes your Favored Enemy from a species to an individual.   The limitation is that you must now take a free action to  mark someone, but you get more to use against them in combat.

 Favored foe still uses your bonus action to cast Hunter's mark.

I would also argue that casting Hunter's mark with a slot instead of the feature does not make them count as a favored enemy.

The first sentence ties the hunters mark spell to the term favored enemy. Not "this feature" or the uses it grants, the spell itself. Doesn't matter how you cast it, the target is your favored enemy.

Favored Foe replaces Favored Enemy but stacks with Foe Slayer