Some early PC compatibles were based upon the same backplane/card approach as the S-100 computers. I have a Zenith/Heathkit Z-160 transportable in my collection

Some early PC compatibles were based upon the same backplane/card approach as the S-100 computers. I have a Zenith/Heathkit Z-160 transportable in my collection. It has an 8 slot 8-bit ISA backplane and everything including the cpu is on ISA cards. This backplane is shared with the Zenith/Heathkit Z-150 desktop. They also released AT class desktops, by this time the portable PC form factor had become too small to support this approach, which used a 16-bit ISA backplane before eventually abandoning the approach for their later computers. Most likely for all the reasons you point put.