Since they show up in most competitive Skaven lists and have a bad reputation: are Plague Monks overpowered?
Breaking down the damage (Pair of Foetid Blades, vs 4+ AR, damage per model)
with Bale-Chime (without) 100 pt efficiency (without Bale, cost 300)
Base 0.56 (0.50 ) 8 (6.7 )
30+ 0.80 (0.74 ) 9.4 (7.5 )
+Charge 1.20 (1.11 ) 17.2 (14.8 )
+ Filth or Rabid 1.60 (1.48 ) 22.9 (19.7 )
Both Prayers 2.14 (1.98 ) 30.6 (26.4 )
Picking Staffs is better if you can get the charge or buff them. However they are very reliant on the charge, annoying because of all their rolling and less reliable due to no inherent rerolls.
How does this compare to...?
I don't like comparisons outside of home armies (that's how they are supposed to be balanced), but their damage output on the charge is about the same as a fully buffed unit of witch aelves before Mind Razor (which almost triples the damage).
Inside Skaven the only real comparison is with Stormvermin: (vs 4 + AR, damage per model)
damage 100 p efficiency (if cost 400)
Base 0.44 3.9 (4.4 )
30+ 0.74 6.6 (7.9 )
+ 1 attack 1.1 9.8 (11.1 )
They have some things going for them, in particular 2" range and retreat and charge. However they are vastly less efficient points wise. A 4+ armour save just isn't worth this difference.
Most mixed lists use them without buffs, since a Plague Furnace means a 200 pt investment. It is a decent combatant, but will get eaten by most dedicated monsters/hammers.
Monks are glassy as can be but work in Skaven lists because of Death Frenzy. They are balanced around their need to get the charge and their 20/30+ unit bonuses and having (mostly) no rend. The main issue is not that they themselves are amazing, but that they are annoying. A box full of dice, rerolls, 4 separate piles of dice due to two "on 6 rules", (2 weapon profiles if you use Staffs.
The solution can't be to just flat increase points since this just hurts pure Pestilens (who have no other choice) and Nurgle (who need all the love they can get). Rather removing the Bale-chime option lowers damage by ~8% (Foetid Blades vs 4+ AR) and particularly removes two dice piles.
6s on hit should feel like something special. Double damage does this, - 1 rend mostly feels like a waste of time. The good news is that you can do this yourself since it is an optional piece of equipment, the bad news is that instead of a rework of the warscroll there will most likely be another point increase.