Ballrooms are often horrible places to shoot. The house lighting is often poor, of the wrong color temp, there are sometimes windows and backlighting to deal with, and/or placed in unfavorable places.
Or there are panels, grilles, lights, and other distracting junk on the walls behind the speakers, which should have been addressed with pipe and drape, but these nuances are often overlooked these days.
The sound systems are a crapshoot. You may or may not be able to tap in, and half the time, the feed is so full of noise you might as well run your own parallel system. Ambient audio and noise is highly variable.
Back of the room telephoto shots are okay if the speakers are welded to their lecterns and never roam from it. But if you have to track them as they perambulate across the room, the long throw gives away every little bump and wiggle while tracking the speaker. Depth of field and shot compression by the long lens will also mess with shot composition, cropping out the distractions or irrelevancies while getting the key images can be more difficult. And some tall goof will always walk thru your long shot at the wrong moment, unless you use risers. I concur with Peter’s suggestion of a dedicated ENG style camera over DSLR’s. Budget for wireless lavs on all speakers, and plenty of batteries for them.
So fight hard to get a second camera closer-in, preferable you want three cameras but a good team who knows each other’s moves and who have experience at this can make two cams look like three or more, by carefully overlapping when they make their angle changes so something is always covered. If you can’t put a manned camera up closer, try for a PTZ job, controlled remotely.