What are the disadvantages to using an MCP2551 as a CAN transceiver for GMLAN / SW-CAN?
The problem is, while the shape of the waves is the same, the voltages used to transmit the signal are different. The MCP2551 is a high-speed CAN transceiver. It uses a differential signal whose recessive state is 0&nbsp;V and dominant state is 2&nbsp;V. Single-wire CAN is a one-wire signal whose recessive state is 0&nbsp;V and dominant state is 4&nbsp;V.

I'm guessing that by "the appropriate wires" you mean you used the CAN-high wire to drive the SW-CAN bus. This wire has a recessive state of 2.5&nbsp;V and a dominant state of 3.5&nbsp;V.

What's probably happening is that these values happen to be within the tolerance of the other nodes on the bus, but you can't guarantee that. Using the MCP2551 might happen to work in this application, but it is not a robust solution for a tool to be used across multiple applications.

Side Note: The MCP2551 is not recommended for new designs; the 2561 should be selected instead.