It's been made pretty clear by many people that Dawnguard feels woefully unfinished and rushed. I'm not going to cover all the reasons why but the actual Dawnguard faction is probably the worst of any you can join. You will ultimately do 99% of the actual work, the members you have to waste time recruiting do nothing to warrant the extra work, the rest of the faction only puts in any remote effort in the final battle- and they barely matter. And if ANY faction warranted you becoming the leader, it would be this, because Isran is dead weight. You will be named arch-mage for doing little work and learning... I think 2 spells at most for the College quest regardless of not being qualified for the role, you're suddenly the Harbinger when others in the Companions likely have the seniority to gain the title... and even if you side with the Volkihar vampires you become the new head of the castle when Harkon dies. Even when it's all over with, the entire Dawnguard stays at the fort and dumps all the smaller jobs on you whenever you show up.
Avarti probably covered that and a lot more in his video on the Dawnguard being idiotic.
That brings me to Isran. He's a narrow-minded, unstable knight templar focused on one goal and little ability to understand anything that differs from his narrow goals. He's got severe blinders on because of his past watching vampires kill his family- and while MOST vampires in the lore are bloodthirsty, evil and have one-track minds, he's not really capable of understanding that some vampires aren't like that even after witnessing evidence to the contrary. He only eventually acknowledges Serana as "different" after she assists in killing Harkon and it's clear that he only doesn't kill her when she arrives at the fort because she's got the Elder Scroll and necessary information- but calls her "it". Even most of the other Danwguard members openly dislike him. I can only imagine how enraged he'd be if he ever learned of Count Hasildor(honestly, this DLC would have been an excellent excuse to bring him back)
I should point out that, lore-wise, while vampires are a certified threat, it's rare they're a major threat. Harkon's plan is an uncommon instance of vampires actually trying anything on a grand scale and even on the vampire side most of them think his plan is too insane and dangerous to work right. For the most part they're no worse than bandits, Necromancers, wolves, Falmer, goblins, trolls, ogres, giants, those car-sized spiders... Isran also seems to be unconcerned with the Civil War, the Thieves Guild, Dark Brotherhood or the dragons, not to mention the Thalmor, who have probably done more in 100 years to harm Tamriel than vampires have in a few thousand.
And if, at any point, you become a vampire, he'll turn his back on you, as will the entire guild, until you get cured. No matter if you've already done the full quest, multiple radiant side quests, etc. Even if he's already somewhat accepted Serana and lets her walk around the place as a vampire, he gives you no such regard. Even if all you stick to for feeding are the bandits and necromancers that you probably already kill anyway if you're a heroic character. It's not like any of the other guilds seem to mind when you have glowing eyes, so long as you're not breaking any laws.
He's a poor leader that's too arrogant to see the world around him, does little actual work from the point you join and refuses to waver in his narrow views no matter what you or anyone else does to warrant better treatment. Dawnguard is in long overdue need of an overhaul mod, honestly.