Allrigth back at it another day; I tested the Death Knight feet path as I am by default a very big fan of black knight style characters (Fire emblem fan here), Kaiju Druid as well and Pestilence Cleric. I'll preface this with the fact that I feel personally attacked by WOTC as ive had google drive full of Homebrew subclass themes for a year, im currently interrogating my playgroup as to who leaked it to WOTC.
Lets start with flavor
I personally have always thought Pestilence is a Warlock thing, but im fine with Cleric getting sum of it. My thought n particular is a big Mosquito patron that gives you power akin to 3E Warlock. Regardless I think Plague Cleric flavor is awesome sauce. 8/10
Hell Knight is cool flavor as a Devil themed fighter. 10/10 flavor (even if stolen from my google drive XD)
Kaiju druid is not in my google drive so I'll say WOTC wins on this one. But man is it sum awesome sauce, 12/10
Demon Sorcerer is p good, ive thought for a while we need more demonic style inso for sorcerers as a juxtaposition of Divine Soul. I think this one is a little weaker cuz it's pretty generic effects that I think should be more fleshed out like having more to do with summoning. Regardless I like where its headed but it NEEDS more 7/10
Death Knight is awesome sauce flavor, but it being a sort of feet/prestige class/weird thing weakens it for me. 6.5/10
Litch.... I'll say it, its not my style. But I do know a lot of people REALLY like the turning into pitch stuff, as such ive had to dabble in it, ill recommend players to stick to grim hollow Litch transformation as I don't think this one manages to implement the vision of the pitch my players had as such ill give it 5/10.
MECHANICS
This UA as a whole had mechanical flavor that was good I like the direction WOTC is going flavor wise, but this batch as a whole needs a lot of work to convince me of its power.
Pest Cleric... is surprisingly (or unsurprisingly for a caster) the best of the bunch. It has a decent level 3 feature that lets it change poison dmg to necrotic and lets itself gain resistance to necrotic/poison and a little flavor in letting it not be infected by magical conditions. Its spell list is allrigth at best and its channel divinity is a little to small for me to view it as a big buff. level 6 gets the bulk from the power as it gets a reaction so that when a creature dies you can pop a use and either incapacitate someone or deal 3d6 (you can guess which one is better). level 17 is... decent? Its immunity to conditions and resistance is good and turning into a little vermin is cool, but dealing 10-15 damage to creatures by walking on their space isn't amazing but its something, the main problem is you have to take the disengage action for this to even attempt to be worthwhile, so not great.
Overall my fix is the aura shouldn't give exhaustion and should instead give the poison condition and ignore immunity to it. Also expanding the aura by 10 feet wouldn't hurt. I would also say maybe making it so you can switch radiant to poison/necrotic would be a good minor buff. The reaction docent need a buff, it's great as is. The vermin... needs a bit, I think giving automatically activating the channel divinity upon transforming the first time would be good, and have it not provoke opportunity attacks.
Hell Knight... Mannnnn this needs so much help it's hilarious. The class takes 2 FETURES TO EXPLAIN ONE. Hellfire weapons and infernal wound NEED to be one skill for clarity sake. Also clarifying that you have a number of dice equal to your con rather than (1 dice equal a use of the skill in which you have a CON amount of uses) as it made me have to read the ability for a while to understand its uses. Even then it's pretty bad, this feels like a poor mans blood hunter. The dice being 6 is cool and I think it needs to stay, but I definitely think that it desperately needs more uses and for those uses to actually have impact. As a starting point I think Hellfire weapons naturally dealing 1d6 additional fire dmg could be a good buff and having it add its con or use con for attack/dmg rolls could be a good minor buff. if they don't want to do that, having the 1d6 tick at the end of every creatures turn would be much better. Also removing the condition for it to leave on healing is DESPERATELY needed as having a class feature just be entirely negated by healing 1 HP is terrible. level 7 gives u resistance to fire and ignoring fire resistance on attacks with is allrigth. I don't think this in particular needs any buffs. Now ADVANCED WOUNDS NEEDS TO BE CHANGED. My subclass feature working 1/6 times every round is not good design, I think a good fix would be to have it always work, and when you roll a 6 you can instead pick two options, the alternative is having a table of effects that allows to have a random effect on roll. I also think that at this level doing 2d6 extra fire damage per attack would be good. The wound themselves are good but rocking so little of the time makes them unworthy unless changed. level 10 gets you an action surge buff with is cool but needs a lil put me up, Making it inflict a wound upon failing the saving throw would be my suggested course of action. level 15 gives you a chance based damage buff, which I think changing to giving you an additional dice of damage when attacking a wounded opponent (3d6 against a wounded opponent) would be good, and for the chance, having it proc the advanced wounds multiple times in a turn could be cool. Level 18... A RIBBON FETURE, fine I guess, the actual future doubles your chances of inflicting wounds and your subclass working... by making you count 1 as 6 on the roll. Yes this also needs to be changed, I honestly don't even have any ideas so WOTC js figure it out.
Now where was I, AH YES TITAN DRUID (Legally distinct from kaiju druid) I genuinely think the titan forms are pretty decent and just needs minor buffs. Giving Behemoth resistances and AC to turn it into more of a tank and giving it more Temp HP would be good. I also think the grappling needs to be at a lower level to be worth. And finally at level 6 giving the option of replacing one attack with either a use of the Titan skill or a Druid cantrip you know could be incredibly game changing for this class, especially since I think it would still be In theme. Giving it some minor damage buffs to the natural weapons like 2d6 at level 8 etc... would go a long way, also giving some extra damage for attacking small creatures could be good. Overall very solid.
Demon sorc is a decent addition, my main complaint is I think the lasso needs to do slightly more damage (Maybe a number of d4s equal to proficiency mod?). And that they need to have more summons. Overall not much to complain p good.
PATHS:
I'll be real, I get everyones hype, but I think this needs a major fix, this is 5es attempt at prestige classes, and to be honest it's not great. Prestige classes in 3/3.5e worked by having the foundation of an existing class and expanding on it in a particular direction. AKA your progression was unhindered while pursuing it, only requiring meeting "lore" and stat requirements. I like death Knight buts it's literally a Paladin at home. At that point make it a paladin subclass. It also hinders any other build idea you have by not allowing you to take feats.it does not have an adaptable spell casting modifier and it consumes your action/bonus action. And as ive heard some of the arguments go that its good for classes that don't have much to do with those actions, that exposes a much bigger weakness in the system that is not to be tackled in this post. I have particular takes about every feat but this post is already long. As such my closing thoughts on it are that I think it needs to be a separate progression that allows you to still flesh out a character build and the effects need a slight readjustment to allow your base class to function.