I've always liked these guys, and that art is spectacular. 

I hate that they are just giving out poisoned with no save, it punishes martials to an extreme degree, but Rogues are just getting screwed with almost fully losing sneak attack while poisoned. Also why the hell did it lose 3 AC? It gained some extra damage, but dear god did it become much more fragile with only +19HP but -3 AC. Overall, probably a buff with the auto poison and extra damage but losing the AC evens it out.

Quote from theron >>

I hate that they are just giving out poisoned with no save, it punishes martials to an extreme degree, but Rogues are just getting screwed with almost fully losing sneak attack while poisoned. Also why the hell did it lose 3 AC? It gained some extra damage, but dear god did it become much more fragile with only +19HP but -3 AC. Overall, probably a buff with the auto poison and extra damage but losing the AC evens it out.

 I've seen you post beneath over a dozen monsters in the MM, and you're always talking about what you "hate." Maybe try liking things for once.

That said, certain monsters pose a greater challenge to characters that attack in melee while others don't. However, it's not really an issue here. The Bone Devil can make one sting attack a turn. Pseudodragons can do the same, and they can also poison (for 1 hour too, though not prevent hit point restoration). As for AC, monsters that can fly that also have high ACs often punch above their intended weight-class. So, lowing the AC of the Bone Devil allows more parties to encounter it without creating a problem for the unsuspecting DM.

Quote from theron >>

I hate that they are just giving out poisoned with no save, it punishes martials to an extreme degree, but Rogues are just getting screwed with almost fully losing sneak attack while poisoned. Also why the hell did it lose 3 AC? It gained some extra damage, but dear god did it become much more fragile with only +19HP but -3 AC. Overall, probably a buff with the auto poison and extra damage but losing the AC evens it out.

 reduced AC is compensated by flight and the auto-poison stinger attack. poison is also an incredibly easy to resolve status effect, and even if the party doesn't have a way to resolve it, the poisoned condition only lasts until the beginning of the devil's next turn. Not nearly as big of a buff as you're making it out to be.

10/10 art. Amazing work i loved the old one and this one is even better.

For stats, as already discussed above the bone devil lost 3 AC, which is a lot but got 19 average HP gets +7 initiative and got a str save bonus.

His claws deal 5 more dmg on average and his sting does 2 piercing and 1 piison more. Makes a total of 13 dmg more. Als they got auto poison which stops healing now!

For a flying monster this is a fair trait and i am excited to test them out.

Quote from theron >>

I hate that they are just giving out poisoned with no save, it punishes martials to an extreme degree, but Rogues are just getting screwed with almost fully losing sneak attack while poisoned. Also why the hell did it lose 3 AC? It gained some extra damage, but dear god did it become much more fragile with only +19HP but -3 AC. Overall, probably a buff with the auto poison and extra damage but losing the AC evens it out.

 If you have a problem with this, give your players antitoxin 

Quote from theron >>

I hate that they are just giving out poisoned with no save, it punishes martials to an extreme degree, but Rogues are just getting screwed with almost fully losing sneak attack while poisoned. Also why the hell did it lose 3 AC? It gained some extra damage, but dear god did it become much more fragile with only +19HP but -3 AC. Overall, probably a buff with the auto poison and extra damage but losing the AC evens it out.

Oh hey, you again. See its funny: you literally answered your own question in your reply. The BD gained damage, a lil HP, and an auto poison at the loss of AC. That's it, that's why those changes are the way they are. 

I'd like to inform any commenters that this is the same guy who commented under the Ice Devil's stat block and, from his own admission after I pointed it out, misread the Ice Spear ability that he made multiple paragraphs complaining about. I really dont trust Theron's judgment on balance. Especially since he thinks 1 Ice Devil is a TPK for a level 20 party... his own words. 

As a last message to Theron: the stat blocks I see you complaining on are all higher CR monsters. Those monsters are SUPPOSED to be a challenge. Like in your Ice Devil post, you said your party had an issue with 1 devil at level 15+. In that case, I just don't think your players are made for high level play. Its not for everyone.  Most DM's are desperate for challenging creatures in higher tier play. At the end of the day, I recommend editing stat blocks to suit your own party. Then when they get get good enough in combat, you can use these stat blocks as is without having to manually power them down. It'll save you alot of prep time. But maybe until then, they should stick to level 1-8.