I personnaly have a flat saddle I use for dressage, jumping and XC and I love it. I feel closer to the horse and it is easier for my legs to go down and be at the right place. I've also noticed that my horse feels better with this saddle but I thonk it also depends on the horse.
The only two disadvantages I see is that it is harder to adjust the girth when on the horse and not all exercise rugs fits in.
But here again the most important is to fond a saddle that suits you and your horse and the best way to find out is to try out some and find the best one !