The Decathlon underwear that I use are supremely comfortable, I've never noticed a seam problem. The material is elastically and they grip not just around your waist but all over their surface area as well. It is important to get the right size though, I'm a Medium (32" waist) but for hiking, I wear their Large size, its support and comfort I'm looking for most of all. I wear the Medium size for running, the grip is a bit tighter but at £3.99 a pair they are really good.
The other benefit for me when hiking is, I wear one and the other one is washed and drying. They hand wash so easily and dry fast as well, the material they are made of does not absorb much water unlike merino and bamboo. As stated the downside of my boxers is if thay are too tight they can be a bit hot (which is why I wear the Large size which solves the problem) and the odour control is not as good as more expensive ones but once off and dry there is no odour problem at all.
I must admit on a hot sticky day when I'm sure no one is around I can just drop my trousers for a few minutes and the perspration around that area quickly goes away and I'm cool again. I can't do that with my marino or bamboo boxers, it would take too long.