Charge Injection in Charge Pumps
Charge injection may or may not be a problem, depending on the performance required of the PLL.

Using two devices switching in antiphase, and making the assumption that an on-going device generates the opposite charge injection to an off-going device, gives you a first order cancellation of the injection current.

Uncancelled charge injection can be regarded as a frequency dependent error current. Where a PLL is used in a fixed frequency synthesiser, it can usually be ignored, as it results in a fixed offset. In a demodulator, the frequency dependent offset may simply result in an effective gain change, which is not usually a problem.

If the charge injection is temperature dependent or power supply voltage dependent, then it can give rise to more temperature drift or sensitivity to power rail fluctuations than you can tolerate.

In a fractional-N synthesiser, it's not usually tolerable, as what was frequency dependent in a fixed synthesiser is now data dependent, and different count sequences that should have the same weight now end up with different weights, causing non-linearity and down-mixing of the high frequency variations into the baseband. In fact, fracN synthesisers tend not to use charge pumps.