In short the primary problems with medium mechs at the moment is that most of them are far too slow to be optimal, SRMs being absolute garbage, and every single fast medium (with the exception of the BJ-1X) is over sized (this is the case for all weight classes, see Awesome, Quickdraw, and the Dragon).
With regards to mediums being too slow please see the charts on the link below.
Speed vs Weight Optimization
From the charts you can see that with a STD engine mediums are the best choice if you want to go 6/9 (106 kph with speed tweak) speeds. In addition, due to the change from 0.5 ton JJ to 1 ton JJ at 60 tons a 55 ton 5/8/x mech will have the most free tonnage with that movement profile. Currently the trebuchetes, CN9-D, YLW, and BJ-1X are the only mechs capabable of having speeds in this range.
Once you switch to XL engines the cicada can carry the most tonnage at 8/12 speeds (142 kph with speed tweak) however, this half ton advantage over the jenner/raven is in fact negligible due to the poor hard point selection of the cicada and it's excessive size. The 50 ton chassis can carry the most free tonnage of any XL engined 7/11 (125kph with speed tweak) mech. However, only the CN9-D and TBT-3C are capable of mounting the required XL350 engine
The upcoming Shadowhawk will be the king of jump capable mechs moving 106kph due to reduced JJ weight. In addition the trebuchet also beats the the quickdraw in tonnage efficiency at 6/9 speeds so long as 3 or more JJ are added.
TL/DR: Fast mediums and quick jump capable mediums are pretty good if you have a team that can capitalize on the speed and maneuverability however most mediums are too slow to be any good.