[rtho782]

Flat cables are generally unreliable as they cannot be fully twisted.

Many consumer electronics devices only have 100mbps ports - e.g. my Sky Q boxes are all 100mbps, Hive, Hue, etc etc. It's more common than you'd expect.

You can in theory run two 100mbps connections over one cable with a splitter, as each only uses 2 pairs of the 4, with an adaptor at each end, but don't do it, it is bad.