It's possible to farm mushrooms to gain 99 in several attributes, limitless wish-level speed boosts, knowledge of every skill and spell, limitless PoCC-level corruption removal, limitless PV, limitless HP, and limitless PP. They also drop quite frequently, so massive rewards can be obtained by investing very little effort.
I suggest the following changes be made:
1. Make some of the more powerful mushrooms only be dropped by the fungoid overlord. The overlord would drop 1d3-1 of each of these types, and they would otherwise only be obtainable by wishing. Mushrooms that should be limited in this way:
- mushroom of indomitability (increases and boosts willpower; doesn't respect potential)
- mushroom of hardiness (increases and boosts toughness; doesn't respect potential)
- muscle mushroom (increases and boosts strength; doesn't respect potential)
- mushroom of the glow (increases and boosts appearance; doesn't respect potential)
- mushroom of resilience (increases PV)
- mushroom of ecstasy (boosts speed)
- mushroom of purity (removes corruption)
- mushroom of inner light (grants skill/spell knowledge)
2. Make some of the farmable mushrooms weaker:
- mushroom of immobility - cap the strength and willpower gains at 20.
- mushroom of the swirly mind - cap the learning gains at 20.
3. Halve the drop rate for the farmable mushrooms. I think this would give about the right effort-to-power ratio for the remaining limitless mushrooms.