Okay, must be time to chime in here. There are two ways to look at this picture.
The first way is to say "Does this stay true to it's 1E counterparts, is this a true and good re-interpretation of the horrors I lived so dearly in 1E?" If I ask myself that question, the answer I get is "The Balor comes close, the Marilith was a good try that failed badly"
The second is to say "Are these creatures well-rendered in and of themselves?" And, that's where I'm going to put the meat of this post, because as an artist I feel that to do any illustration justice you have to ask this question seperately of the first.
Now, I try to keep in mind (because I suffer from it as well) that artists have good days and bad days. Even the best artists put out crap, and the worst put out nice pieces on some days. Deadlines don't distinguish between pics you're proud of, and pics you wish you could just re-start, sadly. That being said, these are my impressions of the Balor and Marilith...
BALOR: From the neck up this guy looks pretty good. Well-rendered, good anatomy, very good use of a single color-family without losing it all in similar shades that just look flat. Nicely done there. The wings are good too, they look like they should, and are about the right size. The loincloth.. I personally would have opted for a bronze-ish hanging armor-esque piece that added some decoration to the issue of a Balor's clothing choices as well as fitting into the brimstone and hellfire motiff of a Balor, and helped protect his infernal naughty-bits to boot.

. The tattered cloth thing really was a mistake, I feel. And I swear if I see one more lightning-shaped sword I'll retch.

The head... had a good start. It's shape is good, it's monstrous and demonic (although if it had been bigger (not the displayed pic, but the size he likely drew it at) he might have been able to give it more horrific facial features, but you can only ask for so much. It's the horns, the horns.. the horns throw the entire thing off for me, because I can't stop looking at them, and I dislike them. They look like they sprout from the top of his neck, and they just aren't going to be helpful to him. In fact, the only thing those horns
are likely to do is prevent him from having a full range of motion with his head. He'll likely never be able to gore with them. That aside, this is a pretty darned good Balor. I'd have liked to see the whip (I am sure I remember Balor's having firey whips) too.
MARILITH: Where to start, this thing was a nightmare. From pelvis to collarbone, she's a very shapely lass who'd turn a lot of eyes in sillhouette. She is, actually, not skinny - that's an illusuon, partially fostered by the fact that her head is too big for her torso (unless she's built on the model of a 10 year old girl). The choice of which weapon is in which hand appears to have been given no thought whatsoever on our right side, because those weapons are in the wrong hands. Spear-scimitar-katar would be logical (spear in top hand, it needs room to maneuver, scimitar in middle hand, where it threatens the most, and katar in bottom hand, where it needs the least amount of movement to be effective). On our left, the worst I can say is that that is the stupidest flail-design I've seen lately.

But at least she's holding her weapons in some logical order on that side.

Her head.. is just silly. There's obviously a
strong push (perhaps from the AD?) to extend her snakelike resemblance up to her head, which I feel is a mistake. Yes, overall this is just a very impressive 6-armed Yuan-Ti woman and doesn't say Marlith to me at all, and I think it's because of the spreading snake-ness. From head to tail-tip she's entirely purple. And while I adore purple, it's too monochromatic (body-wide) here, where the Balor's use of a single color-family (red) worked for him. I have to squint very hard to even notice she HAS hair, or a nose. But, I think where the artist really droppe the ball on his foot was the snake tail. Not I can say all I want about 3E art being too cavalier about itself, and not trying hard enough to maintain it's own internal quality consistency, but many of the snake-like tails I've seen in 3E art have been excellent. This one, is poor. It's technique is poor, it's rendering is poor. I have the suspicion that if the artist were here, he'd tell us he didn't like that tail. If this is Sam Wood's work, then I think he must have either been having a bad art day when he did the tail, or was just bored of the pic by then. To really see the problems with it, you have to see the outline of it, not the texture and color. Visually reduce it down to just an outline, and it's really little more than an inconsistant, blobby tendril. The attempt to do it right was made, but not maintained throughout, and by the time it had been finished, you can see that it'd been lost. Somewhere in the initial sketch stages, I'll bet you that was a fairly alright snake tail.
And, lastly... what in god's name happened to that spear?!?!?

It looks like it's been run over by a few semi-trucks. hehe
It's good work, it's just not excellent work because of some glaring problems that keep me from appreciating the rest of it. And I think the marilith completely lost the "idea and feeling" of what a marlith is.