Too many brushes
Oils and acrylics are less familiar to me than watercolor. I know that you must keep your oil and acrylic brushes separate and not use the same brush for both. I've observed and been advised that it's not a good idea to use bristle brushes for acrylics because the bristles swell up with water and get floppy. Also, they aren't made to be soaked in water and may come loose at the ferrule. So it appears that bristle brushes are the choice for oil painters, along with some sable and other soft hair brushes for blending.
Acrylic painters are advised not to use expensive watercolor brushes, which are easily damaged if not properly cleaned or if used for scrubbing paint onto canvas. Even when used in an aqueous manner, acrylics are hard on sable and sabeline brushes or any brush that is a blend of synthetic fibers and hair. Synthetic brushes are the best choice for acrylics, with varying length of heads and long handles for oil-style painting and shorter handles for watercolor style. You must keep your brushes clean, no matter what medium you use. You can partially restore a brush that has been left to dry with acrylic paint or medium in it by soaking it in isopropyl alcohol and washing with warm water and soap, but you can't bring it back to its original condition. So be sure to keep your acrylic brushes damp while working and clean them thoroughly when you're finished for the day.
I could paint in watercolor forever with three brushes--one-stroke, round and rigger. I learned to paint with a light oxhair 3/4" flat one-stroke brush. This brush, with a head about 1 1/4" long, was the master of all strokes. You could do amazing, streakless washes, beautiful drybrush on the right paper, and even fine lines with the corner. You literally didn't need another brush to do a good, spontaneous transparent watercolor. Now you can't get anything even close to this brush in the catalogs and art stores. Wash-brush heads are much shorter and made of a blend of fibers that doesn't hold as much water and paint. This is why I have one-stroke brushes custom-made for my classes. For finer detail I use a #6 or #8 sable round brush. A good sable brush holds a lot of paint and still makes a beautiful fine line. Many artists use synthetics now, which do detail nicely, but most don't hold as much fluid as a good sable brush. I like a #4 or #6 script, liner or rigger brush to make long, thin lines. The only other brushes I use quite a bit are a striper (a funny-shaped brush that looks like a dagger) and a hake. The striper holds a lot of paint and water and makes a line that goes on forever. I also use a 2"-3" hake brush to dampen my paper or make big gestural marks on a large sheet.
I have just a few synthetic brushes--flats. I use them for exploring paints, when I don't want to flood the small patches of color. Synthetic flats are great for getting straight edges on architecture. They have other uses, but they don't make good workhorse brushes for me.
I've tried to boil the subject down to a few personal observations. If you have suggestions, I hope you'll comment.