Get 5 skelly spawners in one area set up in a fall damage configuration (the fall damaging them to 5% HP) and EXP becomes incredibly trivialized. Even more then it is already.
That's an awful lot of work (and afking within 16 blocks).
In general, modded minecraft balances out as "work required to design is profitable"
And that's before adding mods too. Factor in things like MFR Mob grinders and EXP catalizers form LXP and player placeable spawners start to look broken. You can also double dip if you have both MFR and LXP
On a server, the 16 block player distance effectively makes any system relying on the output of such Broken as Designed, as it will quickly run out of product when the player logs off and thus have overall output far below any alternative. Also, the double dipping from MFR & LXP is fixed (except in a couple edge cases I won't disclose)
Even if you exclude the obvious usage of soul shard spawners, you can generate endless mob essence by using an auto spawner spawning slimes, leading to a drop trap to split it, and then the output of the drop to a grinder. As far as I'm aware this is Working As Intended because of the design effort involved.
(Auto grinder in normal mode (cost 150) with a single drop trap: Large becomes 4 medium, or medium becomes 4 small, generating 400 essence (small dies with no essence generation).
Or grinder in exact mode (cost 500) making large slimes, leading to a double trap large->4 medium->16 small, grinding for 1600 essence. The second design is faster, the first smaller.)
Also if you use Infernal mobs you can end up with several diamond chests full of enchanted chain/iron/diamond gear you can reduce in to handy materials and also enchant books with the LxP isolator and that MFR one.
Sounds like a problem inherent in any spawning ability brought about due to that specific mod.
There is also that Magic generator you can pump exp in to but as you said there are better methods of power generation so that's a moot point.
And only relevant to this topic if you have both Dartcraft and Gregtech. Thus, not relevant in FTB.