I will start by saying that this book commits two sins: it has very little if any understanding of 5e and can't decide if it wants to be high lethal or memorable or wants to be both at the same time (for the record high lethal is synonimus with not memorable, characters can't make much of an impact if they don't stick around).
I will start by saying that I am not the kind of guy that would like to run a meat grinder, I am not the kind that enjoys sending wave after wave of PC to their all but certain deaths but I don't think it is a wrong way to play the game either. Even at the start the game says that this is for a subset of people and not for everyone but I kinda have trouble knowing who this is for, at a glance it is for the said meat grinder people but then they really want at times to make death count and be memorable but yet you can't mourn for too long lets get your backup character ready to be sent to the slaughter.
I will be quaint and just go over the parts of the book, it says to go 3d6 straight down no rerolls. Again in another game this is fine but in 5e it is understood that you are to make characters with some sort of character optimization in mind, even if you roll you are expected to put the highest roll in your main stat (for the record in the PHB you pick race and class first and then you pick the ability scores so you could end up with your main stat as your dump stat). It makes the assumption that somehow low stats will be able to make the game more interesting but I honestly don't buy it at all, being able to do something with a 5 in a stat instead of a 10 will not somehow make things more immersive, it will just lock away choices because they will be not optimal at all, sure you can still try them but there is nothing more inherently smarter about trying to deal with a deficiency, I could have the Barbarian that dump Charisma be the party face and it wouldn't make me a smart person.
Hit dice... just a naked roll based on your class, no Con mods or anything, yes your wizard can end up with a higher HP than your Barbarian and there is nothing that the Barbarian can do about it, yes they are weirdly proud that you can have 5 HP at level 5 where enemies that have five damage for their bonus (that is for later). They say that low HP might lead to good tactics but might as well just play super safe long range characters since any risk might end up with you going down (and you will probably go down at the first sign of unavoidable damage from an enemy spell caster either way). They do say that maybe if the low HP is such a thorn to quest to raise your Max HP... I would say just not mess with the system but that is just me.
The skill and proficiency, here is where the ignorance is egregious. I will just say it, there is no such thing as skill ranks in 5e, you are either proficient in a skill or not and that governs using the proficiency bonus for a skill check. The only thing that they add in this part is that if you are unskilled you don't also gain ability mod if it is positive as opposed to always getting it regardless. It really shows that they did not do their due diligence here.
The Injured mechanic is something that I do like although tweaked to be based on the level of the character as opposed to a flat 10 HP, this can help make things more gritty and dire (but it also messes with the all important speed so it isn't a full on praise). It is one of the few bright spots. Death is also good to use for a more gritty version of the game
Zymer's candle is essentially a save point in D&D that lets you go back to a point in time and the sole reason why you shouldn't feel bad that your characters die all the time because you can just hit the rewind button (mind you as it is written the DM can easily stop any candle shenanigans because it is a candle, I wouldn't feel safe if my lifeline could be severed by an errant gust of wind or interpreting the somewhat vague rules to your favor). Decent mechanic and would help with making things more permanent but still kind of iffy.
The spell section is another blunder of not understanding, I can take it or leave turning spell slots into it all being one time per day use spells (it also doesn't help that somehow you can interpet hat a high level spell caster can have access to the top tier spells in the end, so that a 9th level wizard can rock out 3 level 9 spells when normally they can only cast one level 5 spell, not to mention the 3 spells of a level lower iteratively). The problem is that now spells need skill checks so they don't fizzle out, this is kinda bad because the game already has rolls on most spells anyway and the point of spells being "always reliable" is that there is a limit, of course if you take the obscene amount of spells that wizards can get then maybe this is one wild balancing act but it all sounds ill conceived. Also cantrips are totally useless with this system, better use a crossbow because there is no way that using a fire bolt would be better since you need to roll twice.
The XP system is the biggest point against this entire system. It goes with the old style version of different classes having different EXP leveling requirements, spell casters get high exp costs and fighters and rogues get lower. Except that is not true because Bards have the lowest EXP requirements of them all and they are as magic as they can all get out in 5e; a level 10 bard needs 25k XP to level up, while a wizard needs that amount to reach level 7 and level 10 Bards can have as much magic mayhem as a level 10 Wizard. They really didn't seem to update their conception of the classes from 40 years ago.
Advantages and Disadvantages. They seem to have the impression that 5e is full of bonuses and penalties when it is established that only adv and disadv is the only thing that matters, they are just trying to repackage something that is essentially unchanged in the main game itself.
I'll be honest after this crapshoot affair with the rules I find it very cheeky that they poke fun at the CR rating and yet they also fail to understand it in their own system. Enemies power up linearly in all stats from HP, Attack and Armor, and this is an egregious problem because at the later levels you will have a very hard time hitting anything since all the mooks will have very high armor levels, armor is not supposed to scale up with levels at all. And to reiterate this is for the mooks, the big bads will be encouraged to have more piled on.
Monster AI, I really don't know why does this exist. You can expedite things by rolling instead of actually choosing even though I would guarantee you that picking yourself is the quicker option especially since most monsters have only one true choice, maybe two.
Some words about DMing a meat grinder and how the players are supposed to git gud, the zone system which abstracts detailed area into zones that you can be in, not bad, it is useful for quickness. Same thing with agregate initiative.
This isn't totally befret of merit, there are some good ideas in there so this isn't a total wash but it makes the fatal mistake that somehow deflating the PC numbers means making things more memorable and intriguing. You can easily make a super hard game just with normal D&D 5e as it is written and the book itself even says what you can do to make things harder.
In the end I think 5e makes for a very poor choice in meat grinder (it has the most indepth character creation out of all the D&Ds that I know just because you want a backstory for actual mechanical benefits) and this does not help things at least if you don't want gears to grind to a halt.
[2 of 5 Stars!]