Let's look at the specifications of the beautiful new 2015 Retina MacBook and the Apple charger to help explain why.
The new MacBook includes a 29W power adapter, and when charging it will pull all 29W to replenish the battery as quickly as possible. The USB 3.1 specification states that up to 10W can be delivered to attached peripherals (which then leaves 19W for your MacBook to charge, albeit at a slower rate).
This means that when using other docking stations which rely on Apple's power supply, the docking station itself and everything else attached to the various ports are intended to consume no more than 10W of power. 10W is only marginally enough to power the internal components of a docking station while charging a single modern smartphone!
What, then, will happen when multiple USB devices are attached to this competing docking stations that rely upon your 29W OEM charger? Likely one of three scenarios:
1) As power draw ramps up, voltage drop will occur and some ports and attached peripherals could malfunction and/or not charge.
2) Necessary voltage could be provided to the ports and devices by pulling from remaining 19W being reserved by the MacBook, causing the MacBook to charge slowly or not at all.
3) The MacBook will still attempt to charge, and the power adapter will overdraw beyond specification. Probably goes without saying that this could be very bad.