Content quality is super good. It's a very nice sandbox system for playing with 0 to 2 players (aside the GM).
Yes I said "very nice", not "perfect", because content isn't rich-enough for that for the price (project was kickstarted 18k$), and the "solo playing" rules are not enough elaborated (for that, buying the "Mythic Game Master Emulator" is recommanded ; it's how start the Solo Heroes section).
In a nutshell, Scarlet Heroes is a good mix of different Sin Nomine Publishing : so simplify, it's a typical D&D B/X system in the Red Tide setting with some Godbound rules (the good ones, like Fray Die) to make the characters super powerful ; and an extended "Black Streams : Solo Heroes" content, to play with just 1 or 2 PC, or even without GM.
So if you are just discovering this publisher and his sandbox philosophy, it's a very good entry point. If you already know the others books, Scarlet Heroes will probably appears very classic or even useless.
The writing is very pleasant and easy, always go straight to the point, concise. You should try "Scarlet Heroes : Quickstart" to make you a good idea ; this books add some advises and generators and rules to play solo, but it's more about "explaining more" than adding new features.
About the printing : I guess it went though Drivethru Print-On-Demand (it's not clearly written in the book), and the quality is quite bad : paper of bad quality and almost transparent, hard cover not properly fitting (edge is too thin, giving the book a curled appearance), and I have a feeling the content is not up-to-date with the last PDF version (ISBN is different). At my knowledge, author didn't published a changelog to know what are the differences since.
So sadly, buying only the PDF and printing it on xpress.lulu will give you a better and cheaper result, but unfortunately, less money to the writer.... Or, at least, buy the softcover instead of hardcover.
The book take place in the "Red Tide" setting. It's nicely introduced in this book, but if you want to properly use that setting, I will recommend buying the "Red Tide Campaign" book from same author, if you want to create long campaign with a heavy focus on the universe. "Red Tide" is originally aimed for Labyrinth Lord, but you can easily use it for Scarlet Heroes. Else you can transpose Scarlet Heroes in your own setting.
My only disapointment is the same as for Red Tide : humans are divided in disctinct "civilisations", inspired by : chinese, japanese, nordic... They have there own cultures, so it's kinda sad that it was not taken in account for classes & magics ; would have be interesting to have exclusive classes like "wuxia/wandering knight" for the chineses, samurai & ninja for Japaneses, berserkers/barbarians for nordic... Would have been interesting Clercs and Wizards having different spells depending the cultural God or philosophy, spells based on Tao, Yin-Yang and the 5 elements (wood & metal instead of Air) for ones, and shintoism and anismist for one other, Norse mythology etc...
Instead of that we have very generic D&D magic with some extra spells, and only 4 classes (Warrior Thied Cleric Magic-user). "The Sandbox" magazine (free) introduce 2 others, "The Last Prince" and "Blademaster" giving you tips about how to create your own.
The Scarlet system seems, at first sight, very similar to Labyrinth Lord and B/X, but is, in fact, very different, allowing more powerful PC (you need do compensate their low number !) to single-handle almost all situations, like in Godbound.
Still, uou can easily adapt any D&D B/X books to Scarlet Heroes system (it's explained how to), allowing you to play with a reduced amount of players. Switching from any OSR system to Scarlet Heroes and vice-versa (depending the number of players you have) should also be easy.