"Fleet cheese" can be somewhat defined.
And it boils down to primarily one thing, firing arcs.
From personal experience (mainly from Karnak's free-fleet missions, but it's still experience...
):
An ISC plasma fleet with a lot of FH-arced plasma (CA,CL,DD of various refits) isn't too bad to use, because the trick to using the fleet is to keep the ships in a decent echelon formation and firing before the target clears the FH-arc of the lead ships.
A more PPD-oriented fleet is harder to use, (swap the CL for a CS) as now not only am I trying to keep formation, I need to get the CS to fly the same direction as my CA, in the proper range bracket, and have the target in the narrow band of space covered by the FA arc of both ship's PPDs. Line formation helps out here, but can be a pain because the CS will do "bobs and weaves" if I'm travelling slower than the CS wants to at that moment.
Some of the hardest Karnak fleet-missions I've faced is against droner and Carrier AIs. A droner fleet doesn't care about position or arc, as the drones will launch no matter what the AI's doing or where it is. Actually, droner fleets benefit from a wider formation as a single T-bomb won't take out all the drones. Meanwhile, numerous carriers put so many attrition units on the board that it's impossible to kill them fast enough before the mass required to kill me gets into range (in SFB, it's said that 10 Stinger-2s at range 0 can wipe out a K-B10 in one shot. The trick was getting 10 of them in at one time, luckily the Hydrans probably had 30 that day...
) Putting these fleets into live control only exaggerates the problem, as the live player can coordinate the fleet much more effectively than the computer.
Therefore, "fleet cheese" is a combination of 2 factors, how many HWs can you point at a target at a given time, and how many attrition units can you send at one target at a given time. More attrition units can (obviously) overwhelm the defenses of a given target easier (actually landing hits), while the wider arcs means that it's easier to get more HWs on one target for longer periods of time.