Like Allen said, each style has its advantages and disadvantages. So let me give my (amateur) take on what I think are the advantages and disadvantages on the different methods. These are just my opinions for sake of discussion. I'm sure I missed something. Aloha.
1) Spanish Heel: Advantages: Strong, can dial in perfect neck alignment, traditional, some say beautiful. Disadvantages: Complicated, requires a lot of time and work, skill level needs to be good. I once was going to attempt a Spanish heal and after doing my research I remember thinking; Why? There are many adherents to this method and some of the best luthiers in the world swear by it.
2) Dove Tail: Advantages: Strongest joint, artistically beautiful, traditional, time tested, acoustical advantages, did I say strong? Disadvantages: Difficult to remove, requires medium to high level of wood working chops to pull off well, a little old fashioned maybe. Hey, this is the joint for the serious luthier. Martin & Co. are no dummies and after selling about 100 billion dollars worth of dove tails sold, they just might be onto something.
3) Mortise and Tenon: Advantages: Logical, relatively straight forward and easy to do. Disadvantages: Maybe not the strongest joint. Mya-Moe uses this style with great effect and their ukes are fantastic.
4) Mortise and Tenon with bolt: Advantages: As above except stronger, easier to remove/adjust. Disadvantages: Aesthetically industrial, can move, more work. This is the style on the guitars I own. I love them, but they move which can be weird.
5) Butt Joint with Bolt: Advantages: Easy. Disadvantages: As above except even weaker.
6) Butt joint with dowels: Advantages: Easy, it works. Disadvantages: Inherently weak, alignment issues.
7) Straight Butt Joint: Advantages: Easy, quick, cheap. Disadvantages: Structurally weak and unsound, prone to catastrophic failure. Hey, this joint has made a lot of junk ukulele builders a lot of money so there must be something to say for it. Stack em high and sell em cheap. So what if the neck falls off.
8) The Perfect Neck Joint: Advantages: Strong, perfect alignment, easy to make, removable, acoustically alive, cheap to produce. Disadvantages: None.