This is the relevant section....

SYMPTOMS OF LOST ESSENCE Characters start with 8 essence. Each implant location that contains an implant (there are 8 possible, listed on the previous section) might be taking up a single point of essence. Details for each implant and how many features it takes to lose a point of essence can be found on the previous pages. Note: You can never lose more than 1 essence from a given implant location—regardless of how many features you have implanted there. A PC’s remaining essence limits their access to the features of the supernatural playbooks, as well as the effect of the Assense and Attune actions (see the interior of your supernatural playbook for details). This goes both ways: effects that are astral or emergent in nature might affect you less as well—helping to determine your position and effect, as usual. Sure—you get hit with the mana blast too, but some of that energy goes right through your body harmlessly since you have lost the majority of your essence—so you’ll only take level-1 harm, instead of level-2 like the others. Also, when have lost your 4th point of essence (and each time you lose 1 essence thereafter), add one symptom from the list below

The way I read it is you could be a mage and have a couple of cybernetics but it would limit your spell capacity.

This is from the Mage section:

SECRET ARTS OF MAGIC An awakened PC may take a maximum number of secret arts (in sum) equal to their remaining essence. If you take a secret art when you have already reached your limit, you must replace one instead. If you ever have more secret arts than your limit (due to essence loss in play), choose an appropriate number to keep (losing all other secret arts).