Different tumor types have different tumor DNA shedding rates -- this has impacts on test sensitivity. A false negative is not great when dealing with a cancer diagnostic. Additionally, and more challenging, is the variant classification problem. Not all mutations are created equally. Some are pathogenic, some of benign, and some have unknown significance. If you simply report the genetic alteration, and I believe this is the approach taken by Guardant, the onus is on the oncology care team to determine whether the alteration is clinically relevant and actionable. Companies like Myriad and Foundation Medicine provide guidance on alterations which makes this less of a problem. The alternative is for a caregiver to do homework on the alteration and determine whether it is meaningful (through a literature search or ClinVar database query).
Variant classification alone is an incredibly challenging problem. Oncologists are not geneticists, and most hospitals do not have in-house geneticists. Additionally, there is potential liability involved with mischaracterizing genetic alterations.