It will be bad on water flow, not due to the splitters, but due to the extra tubing lines.
Lets say your flow is 100%, if you split it, it will have 50%, if you split it again, it will have 25% flow.
Less flow is less pressure, and we need pressure to create turbulance in the waterblock, to get a better cooling performence.
Look at it from an other point of few. Are you going to OC your hard drive? does it generate 100W of heat? most likely not. Your hard drive generates 5 to 10W of heat.
When you look at the point that needs the cooling the most, it's your CPU, so you want to cool that the best.
As you have cold water in your system, why not use it also for the rest, AFTER the cpu?
After the CPU, the GPU needs the most cooling, after this your North bridge, and south bridge, Mosfets, PSU, Memory etc.
Your hard drives don't need watercooling, but it will keep the temperatures in your system down, so that will help al stable OC. Also the lifetime of your harddrive will double when they are watercooled.
One other way to look at it-> why watercool all the rest? it will add more heat in your water, so your radiator needs to disipate more heat. If it is capable of handling it, no problem.
If it is not, get an extra or better rad, or don't watercool the rest
If you just want to go for a total watercooled system, and you are not into overclocking, your CPU temps will rize about 5 to 10 degrees, but still will be better then a standard intell air cooling