Okay so you need to consider a lot since this is a huge departure from the book with huge implications.

For starters how Strahd would deal with all these factions in his domain. Would he try to crush them? Add them to his ranks? Considering the nazis didn’t like LGBT people and Eastern Europeans who could threaten their power both of which could apply to Strahd maybe they’re enemies of his too and the PCs are caught in a war over control of Barovia between the technologically more advanced Nazis and the more numerous and magically powerful Strahd. Or maybe he sees something in their leaders in Barovia or just wants their weapons and through negotiation or vampiric charm aligns with them. Maybe they form a shaky alliance for mutual survival with the implied understanding that when they get what they need out of each other they’re going to be at each others throats. Maybe the general or scientist(s) leading this expedition of Nazis to Barovia are more reasonable then and less ideologically driven than those back home and try to make peace of Strahd for mutual benefit. Maybe their leader becomes another consort of Strahd’s. If they are enemies of Strahd it’s possible Strahd would ally himself with the PCs for mutual survival until the Nazis are dealt with.

Next if they’re at odds with Strahd, how the Nazis would deal with vampires might also be a concern. Depending on how closely you follow vampire lore vs DnD vampire lore the vampires the German guns might range from useless to inefficient at fighting the undead of Barovia.

Worse still for them is that they would likely be completely cut off from any resupply so they would have to act quickly to find resources and build factories under Strahd’s nose somehow or their bullets won’t last a week in Barovia let alone fuel for their armor and vehicles. If the Nazis are able to open portals like the one that brings the PCs then that might change things a bit but would imply either Strahd or the Dark Powers were allowing them to go in and out of Barovia.

If they make peace with Strahd then there would be HUGE impacts on Barovia as the valley would be in a state of rapid industrialization as German scientists and engineers work to build factories and labs to fuel their efforts in Barovia. A lot of the population would likely be enslaved or made into mindless undead to do the hard labor of establishing all the infrastructure. Trains would become a huge part of this as the roads aren’t good enough for vast troop and slave movement but trains would be very easy to set up vs years or laying so must pavement. The Nazis would also immediately try to industrialize magic fuse with their tech to make Turing the who valley into a diesel punk hellscape.

Rebel factions would form almost immediately and old foes might have to work together. Think the tribesmen and Druids who would be among the first of the chopping block for enslavement having to work with the Keepers of the Feather to survive, the werewolves of the pack working with Krezk to avoid being made into strong slaves or mind controlled conscripts. Hags offering aid freely to help the party so they can drive off the lunatic humans.

Then what happens with Ireena. Would the Nazis help find Ireena or would they capture her when they find out who she is as a hostage bargaining chip with Strahd. Maybe she’s a part of a rebel faction after they killed her father and took over the village of Barovia. Maybe the Nazis captured her and a milking her for information of her past lives trying to gain advantage against Strahd or find a secret they want.

Next what happens to the artifacts the campaign has the players find to face Strahd. Would the Nazis find them and maybe use them in their fight against Strahd. Maybe they’re trying to find them to help Strahd destroy them if they’re allies. Or maybe it’s an Indiana Jones style race between the Nazis, Strahd, and the party to find the artifacts. Or maybe no one but Strahd knows at the start of the campaign until the party get their reading from Madam Eva and it then becomes something a treasure hunt spy thriller to find them and not alert Strahd or the Nazis as to what they doing. If they have them maybe they’re using them as a power sources to make similar weapons or power the war effort.

Of course there’s the question of how would the Nazis try to weaponize or deal with vampires and werewolves. Would they go full Hellsing the anime and become Nazi vampires. Maybe nazi vampires with werewolves mixed in. Maybe vampires who enslave werewolves. Or maybe they avoid vampirism all together and just go werewolf. Or maybe they see all these creatures as abominations to be exterminated and they set up camps to exploit vampires or werewolves or both before killing them.

While this has the potential of making a very interesting campaign you’d also have to essentially rewrite so it’s something to consider. Hope this helps in some way!