I am a huge romance reader and I am drawing a complete blank at this question. I don't think I ever thought about when picking a romance what obstacles there might be, if any. Isn't that part of the story? So I wouldn't know that up front.
I am more about tropes and themes when I read romance. Maybe I am mixing up the terms here, or I am just confused.
I do know I hate when the characters just don't talk to each other so misunderstandings are there that could have been resolved within 20 pages. Not sure if that is a obstacle or what. I am still not sure what that exactly means.
Other than picking subgenres of romance based on my mood at the time, I also pick by theme or trope sometimes. The tortured historical Hero is always a favorite, or the rake with a golden heart that falls in love without noticing before the reader and you really really believe it. Especially if the heroine is someone that is either a bookworm, wallflower or just otherwise not a fluffy empty head.
I am also clueless what MMC means in regards to billionaires. If I want a rich and powerful hero, older harlequin presents did it way better than most of the stuff out there now. But thats me.
I also don't consider a serial or a non ending series a romance. HEA/HFN in each book. If they are like 3 books, I'll just call it a really really long book if I like the story, rarely though. I don't read novellas either. Just not enough length for me to get the whole romance in there.
I don't think I am much help.