Well, apart from the obvious ones that per the usual UK standards of implementation the lane will be too narrow and will encourage car drivers to pass without pulling out to give sufficient room, and then on the approach to any kind of junction it will (a) veer off in some completely stupid direction (e.g. round the outside of a roundabout or onto the pavement) and/or (b) disappear completely, and it will probably be full of parked vehicles.Seriously, what problem do you have with cycle lanes + traffic speed reduction (confinement to 30mph; aiming for lower)?
Good quality cycle lanes would look a lot like bus lanes. I would have no objection to those, if anyone would like to build some. I like bus lanes.
(This post relates to urban cycling and does not address rural issues)