- A Class 92 in DC mode is limited to 4 MW, whereas on AC it can output 5 MW (and other, more modern four-axle electric locos manage 6.4 MW), and even that 4 MW capability already required significant infrastructure upgrades (I think they basically had to double the number of substations along the designated freight corridors between the Channel Tunnel and London)

- Anecdotally, even with just passenger trains on some heavily trafficked sections of route they're running into the problem that the current draw becomes so high that it becomes hard to distinguish between regular operations and an earth fault (and building even more substations isn't cheap, and if you compare it with the amount of substations you'd require for high voltage AC…)

- At switches and crossings you get unavoidable gaps in the third rail, and especially at the complex track layouts around some of the larger stations those can occasionally combine into gaps of substantial length. Multiple units with distributed traction (which have dominated passenger traffic on the Southern Region for a long time) can handle those better because there are pickup shoes distributed along the whole length of the unit, plus possibly a traction power bus connecting those pickup shoes together (at least within a single three-/four-/five-car unit). So all in all it's relatively unlikely that a multiple unit will find itself completely off the traction supply ("gapped").

With a freight train powered by a single locomotive on the hand, power supply to the locomotive will always be interrupted by anything but the very smallest gap in the third rail, which means that the tractive effort gets interrupted on each of those occasions (and to avoid jerking the couplings you want to turn off and re-apply power smoothly, and especially the former will make driving more complicated), you need to power the loco's auxiliary systems through all those interruptions, and if the train comes to an unscheduled stand within a junction (whether due to a technical fault, external influences like trespassers, a driver error, or whatever…) there is a much higher risk that the train ends up gapped and will then cause quite a bit of delay until you can somehow get it going again (if you're very lucky the track gradient might get the train rolling again, otherwise you either need to connect up some extensions leads up to the loco or else give it a push with another train/loco, all of which takes quite some time to set up).