I've never had an issue with these flat cables personally. I use one myself, the supplied one by BT which is very long (at least 2 metres I would say).
I had it initially plugged in my living room, with the cabling tangled amongst an 8 socket lead. Had the TV, sky box, dvd player, a couple of lamps, ps4 and phone chargers literally touching the wiring. I would still get 14 days of solid uptime without a drop out and remained on fast path.
I then had it at my computer desk, here I had the desktop, router, printer, a couple of switches, toothbrush charger (random I know), a lamp, shredder, again all the wires tangled together behind the desk. Again, I had a full 14 days uptime without a drop out and remained on fast path.
I now have my router plugged into it's own plug socket, somewhat out of the way, with nothing around - but I see no difference worth noting in performance. Certainly my RJ11 cable never picked up enough interference that was noticeable to me in these "bad" locations.
I think often the gains of switching are overplayed, but in my view if something is generating so much interference it is causing issues the device should probably be replaced.