If I were building the perfect custom rifle it would have a blind magazine. The guns are about 1/4 lb.-1/2 lb. lighter and a good quality floorplate would add about $150-$200 to the cost of the gun which I could use toward more important features. Like a better quality barrel or optics etc.
Guns with blind magazines have the potential to be more accurate since the stock is stiffer with no cutout for a magazine or floorplate, and getting the rifle bedded correctly is easier.
A well designed floorplate is my 2nd choice, and most of my rifles have them. If done right, with quality materials they generally work fine, but some, unnamed companies use very cheap materials in all their bottom metal. Even well designed floorplates, made with quality materials will sometimes fail and dump your ammo on the ground. Depending on what you're hunting this could be very bad. Some dangerous game guides have been known to weld their floorplates shut to prevent this.
I'm sure someone will be along shortly to point out that if a floorplate fails it is either operator error, or a faulty floorplate. That is true, but they are missing the point. It doesn't really matter if the floorplate fails because it is poorly designed, made with cheap materials, has a worn or broken part,has a broken latch spring, If ice or debris prevents it from fully closing, if a twig bumps the latch, or if the gun owner simply failed to close it properly, it still failed. A blind magazine won't open up and dump your ammo on the ground. Period.
I am slowly coming around to detachable magazines. I used to not like them at all, but if properly done they are probably as good as any. I don't care for those that extend more than a tiny bit below the bottom of the gun. They also will allow you 1 less round in the magazine with some guns compared to a blind mag or floorplate. They also can cost a lot more to replace, or if you want to own a spare that they should. Most that I've seen cost around $50. That can add up, and if I owned a detachble mag gun I'd have to own at least one to have as a spare. On guns such as AR's where they are dirt cheap, I don't have as much of a problem.