It's a supernatural action series with vampires, vampire hunters, werewolves, Frankenstein's monsters and various combinations of the above in Japan and Russia in the year 1930. It's okay. It has things I liked. It was decent enough and I'm reasonably well disposed towards it, but afterwards I deleted the episodes without a thought and I can't imagine ever feeling the need to rewatch it.
The show's main problem, to be honest, is that one of its supporting characters is outshining the rest of the show. Her name's Ryoko Naoe and she's the daughter and heir of a shipping magnate, but also determined, intelligent and not at all bothered about being in the wrong show. She likes our tormented angsty hero, Yuliy, and she'll follow him pretty much anywhere in her quest to help him and find out what the hell's going on. Vampire attack? Train raid? Chauvinist over-protective 1930s father who believes that daughters should do as they're told? None of those stop her.
I preferred all Ryoko scenes to all non-Ryoko scenes. I have no objection to Yuliy, the Jaegers, etc. but seeing vampire hunters fight vampires is clearly less compelling than seeing a tough-minded but mostly powerless civilian in the same situation. She gets a satisfying amount to do in the first half (in Japan), but then gets sidelined in the second half (in Russia). This is a shame. (There's a little girl in ep.2 who serves a similarly empathic function, incidentally.)
Unfortunately, though, this is an action series. Monsters gotta fight. Vampires will be toweringly and one-dimensionally evil, but often also kind of magnificent in their arrogance. Getting killed annoys them less than the fact that their killers are plebians. "You're a filthy animal without an iota of elegance." (Translation: you're an insect compared with My Snobbishly Vampiric Immortalness.) Besides, the fight scenes are pretty good. I liked the last two episodes, which have a strong, exciting action finale and then after that some meaningful non-action stuff. Ryoko gets a good epilogue too.
The rest of the Russia episodes had been a bit dull, though. The show turns into The Quest For The Big Handwave, which has magical powers that make you omnipotent and capable of doing anything, except when they don't. (Will you save your brother? No, of course not. Not when he's got an emotional scene that looks as if you're about to kiss each other.)
The cast are fine, as far as they go. (In most cases, that's not far enough.) Juliy's a very familiar kind of hot-headed vengeance protagonist and I don't think I ever really found him that interesting, but I still like his emotional growth. At the start, all he cares about is revenge. By the end, he's done a lot of growing up and is capable of converting enemies just by trusting them. (One of the messages of the finale is that people respond to being trusted, yet the baddie's downfall comes from blindly trusting in the Ark.) Of the other characters... Ryoko's great, but ultimately sidelined. There's a secret agent whose name I can't remember. The other Jaegers (vampire hunters) are a jovial bunch of badasses with eccentric costume choices and usually no personal motivation. They do their job. They're good at it. They're fun, but even the show itself ultimately decides that they're not very interesting.
To be honest, everyone who really matters to the plot is either: (a) a monster, or (b) the father of a monster, either real or adopted.
Similarly, the background's fairly generic. The 1930 historical stuff is nice, but again underused. (The Hyakko Party end up just being goons who'd offered the promise of being more.) The vampires are every vampire cliche played to the hilt, although that disease could have been interesting if they'd taken it further. The Frankenstein nutter feels like someone who might have been more important in an earlier draft.
Random nerd observation: the "Sirius" of the title is the name of the show's McGuffin and of Juliy's werewolf family. In the Harry Potter books, Sirius Black is a werewolf-like animagus.
It's a "Netflix original anime", for what that's worth. It's better than a lot of them. It's probably not worth watching over shows that are actually good, but it's decent enough action fare with legendary monsters. The first half's better than the second half.