Vias turn at right angles and cause impedance discontinuity. It is difficult to get a 50 ohm impedance when signal path encounters a via. This can cause a disaster for high speed signals.
When doing high speed design, how do people deal with vias? Sometimes it may not be possible to avoid them, and I am not sure if using termination resistor is sufficient. It may be possible to carry out 3D simulation to determine how signals will behave when passing through a given via, but I am not sure when people would need to go so very far.