Sera always suffered backlash from static character development, people grew accostumed to changing their companion's world views into something closer to their own and when faced with a cuckolander who doesn't give ground they got really mad like those religious people who can't accept others with a different opinion.
Since the DA fandom is more or less divided into a pro-elf and an anti-elf camp, Sera inevitably got flak from pro-elf players who didn't want to hear from an in game character why their beliefs sucked.
Another lesser point of conflict is that she is the only romanceable female Elf and she is a lesbian, male players who aren't into gay pairings could only romance Cassandra or Josephine so they got really butt hurt with the lack of options.
Personally, I really like Sera and enjoy romancing her. What's more interesting about her romance is doing so as a mage, just like Fenris you have to work past a lot of differences to make it work and that makes it all the more rewarding.