Be aware that there have been service difficulty reports mentioning some installations with automatic switches that did not include an annunciator light to warn the pilot that the system was operating on the auxiliary pump. In such cases, the pilot could be unaware that primary pump had failed until the auxiliary unit itself breaks down. An extra set of electric gyros would provide total systems redundancy for your vacuum system. Plug in an electric attitude indicator and you would have attitude information available even if your entire vacuum system were to kink, crumble or collapse. If you wanted to back up both the attitude indicator and the directional gyro, the equipment cost would double.
Installation is the major drawback here.
You would need to find space on your panel for one or two fairly large instruments. Further, they should be installed high enough on the panel to facilitate their use in emergency circumstances. There are a lot of Cessnas flying with ample usable space just to the right of the radio stack. Installation of the instruments there introduces a parallax issue , but the instruments can be used with training. There are a number of other planes flying with the ADF in the upper left corner of the panel.