The episode was well paced, looked distinctive in a not bad way and was otherwise generally well-crafted, though the ending fell a bit flat. The in media res beginning takes a very different tact from the novel, and one that I'm not sure I like, though it's eminently possible they'll shift back and begin from the beginning with the next episode.
A few divergences from the novel were a bit meh. For one, the Named enemy unit dealt better with Tanya in the book, though this isn't as obvious without her narration, and the fight was more tactically described, though text is a better medium to describe the tactical elements of this kind of fight. The fight itself certainly looked different.
A more significant divergence is that Visha was well aware that being in a pill box wasn't really safe from the start, and began to admire Tanya well after she knew what the promotion was doing.
Lets not forget that she started going with prayers in the episode before she was even going to use the ultimate.
The novel makes a point of saying that she prays when she doesn't need to do so. It's explained at length as a PR move on her part, since she's going to break out into random prayer anyway eventually, Tanya decides to try and fold it into her image, since being religious is considered a positive trait and it makes more sense to do it universally instead of randomly breaking out into prayer only at key moments. It's also unspecified how much of the Type 95's output she was using in those scenes, which are presumably from parts of the conflict that the novel skimmed over.
Change of art-style I can somewhat bear (except Visha, she is now a frog person), but the alteration of how they fly, their ridiculous leg "thrusters". Forgoing helmets, the enemy *NAMED* unit of all units not wearing armor or any kind of additional protection... and those RIDICULOUS HORSE TOILET THINGIES they were riding on make no sense in-lore or strategically at all. They only make them a larger target and inflexible one at that.
The manga's an adaptation, and doesn't really depict things entirely accurately either. The enemies are only described as kitted out for a long range mission (which would make some easily ditchable means of carrying extra ammo/supplies/etc reasonable). Likewise there's frequent mentions of balance issues that mages have in flying. There's no indication that anyone uses armor in the novel and repeated statements that Tanya is only wearing a uniform, for the real reason that you don't actually see plate armor in a military environment defined by machine guns and rifles. Otherwise it wouldn't really make sense for Tanya, a valuable strategic resource, to skip on armor. There's no drawing of this enemy unit, and no specifications about what long ranged forces might be using.
There are any number of functions the horses might serve. It's very hard to judge without in depth knowledge of the technology and gear used, something we the audience don't have. In practice they're there to make it easier to clarify which side is which during combat scenes with multiple characters and a lot of movement. They're not particularly sillier than having one side use plate armor which hampers range of motion, visibility and wouldn't provide much in the way of protection against actual WWI weapons, much less the magically enhanced kind of firepower we routinely see even normal mages using. Given that there are already magical means of protection that can stop those things, I'd take the silly looking horses over the plate armor for an embellishment.
It's definitely different from the manga, but the manga is itself an adaptation, and not necessarily an extraordinarily accurate one.
While I agree that there are some issues in the adaptation, it's silly to proclaim the anime a 'disaster' based on one episode aimed mostly to introduce the setting and characters. It's also a point worth noting that many of the things you critique it for doing and not doing are things that don't and do happen in the novel itself.
It wasn't great by any standard, but the anime's hardly a 'disaster' at this point. If anything, the fact that Tanya asked the enemy to surrender before firing instead of freaking out and screaming at them, makes it a more faithful adaptation than the manga up to this point, given that the point of pulling back to evaluate their perspective and draw a lot of silly interpretations was a fairly key element of characterization, especially in contrast with how the enemy actually interprets those statements.
(Granted, the flashback of that scene from camera somehow seems to portray the events that happened in the novel instead of the rather different dialogue they just showed happen in the manga adaptation...)