Definitely. Sirius Black is a character who has seen a great deal of darkness and has a rather wild side to him. Hermione is a character who values knowledge and order. In fanfiction, if you want to bring them together, you need to find a common ground. Maybe it could be their shared love for Harry, or their fight against the Dark Arts. But still, making their relationship work in a story requires a lot of thought. You need to think about how Sirius would react to Hermione's intelligence and how Hermione would deal with Sirius's unpredictable nature. All these factors contribute to the difficulty of writing Sirius Black in Hermione fanfiction.
Yes, it can be. Sirius Black has a complex character with his own backstory, and when incorporating him into Hermione - centric fanfiction, it's tough to balance his personality traits like his recklessness and loyalty with Hermione's studious and rule - following nature. Their different personalities and the existing canon storylines can make it a difficult task to create a believable and engaging plot.
Well, it depends. If the writer really understands both characters well, it might not be that hard. However, Sirius Black comes from a different generation and has a lot of baggage from his past, like being in Azkaban. Hermione, on the other hand, is very much a product of her Hogwarts education. So, to make their interactions seem natural in a fanfiction, one has to be careful about how they meet, what they talk about, and how they influence each other. It's not an easy feat, but not impossible either.