I think if you read the DM comments and take that as your basis that "so many people are against her for stupid reasons" you're forgetting that the DM comment threads tend to be full of anti-Royal trolls. Likewise with comments on articles that get posted on Facebook and the like.
I do think there are some very legitimate concerns about Harry's choice of a girlfriend, but more because of the fact that as an American she doesn't really know the culture that she could potentially be marrying into. It's not her race, her religion, her career, her previous relationships... it's the fact that she's grown up in a culture that encourages the freedom of speech and personal expression and being a member of the BRF means limiting what you say and do when in the public eye. I think it's fair to worry about how successful this relationship is going to be in the long run because of how drastically different her background is from what his reality is.
There is also the fact that this is a long distance relationship. I understand the eagerness for a royal marriage... but they've been together for less than a year (from most reports and the Piers Morgan interview it sounds like it's been tops 7 months), with them being on separate continents for most of that time. We've all seen the disastrous effect that divorce and poor relationships can have on the BRF, after the War of the Waleses, the end of the York marriage, the end of the Anne/Mark marriage... I don't think anyone wants Harry to rush into a marriage and repeat the mistake of his father, aunt, and uncle.
There is also the fact, fact, that if she marries into the BRF Meghan is very likely going to have to give up her career. Royals with jobs have a history of not working out in the BRF. Look at Edward's attempts. Look at Sophie's. Even the not-royal Zara and Mike Tindall, Peter Phillips, and the Middleton family get criticized if they're seen as using their royal connections too much within their careers. And Meghan's career involves her actively engaging with the press and doing talk shows to promote her work - which let's be honest, is going to cause problems, even without her getting married. Do you really think that if she does an interview Harry's not going to come up? Or whether or not she's met the Queen?
And, again, the issue of location is going to come up. As an American citizen working in Canada (where Suits is filmed), Meghan requires a work visa. As a member of the royal family, Meghan would also require an additional security detail provided by the RCMP and paid for by the Canadian taxpayer... which is not going to endear her, Harry, or the BRF to Canada.
I think it would be stupid of Harry and Meghan to rush into a marriage. And I think it would be completely illogical for them to do so while she's still under contract for Suits - I think any marriage has a lot more chance of long term success if they were to wait until after Meghan's obligations to Suits are over and they've had the opportunity to actually live on the same continent for longer than a week.