And while some mobs might fail their RR by 20 pts, if the mobs are the same level as the caster failure is on average much lower. Fireballs will probably do about the same, you can shoot a fireball at a range.
A Fireball has an area of effect of 10'R whereas a Sigil of Pain has an area of effect of 10'R
per level. A Fireball is blocked by obstacles (walls, physical barriers, etc.) whereas a Sigil of Pain's effect is not. Casting a Fireball costs PP, casting the 'spell' enruned by a Sigil of Pain does
not since it's just as reading a rune; sure, having the Fireball on a rune paper would have the same effect but "
Fireball" is a level 8 spell, requiring a Runemaster to be at least level 16 to enruned it and at least level 20 to embed it as a chargeable rune whereas a Sigil of Pain may be cast starting from level 9.
Sure, it doesn't give a Runemaster complete invincibility, as he'd still be vulnerable to projectiles, mostly, but he's a
Runemaster, an Essence spell caster. A
Solid Barrier (from the very powerful, but not enough to be broken, "
Barriers" Closed Essence spell list in the SUC) is a level
9 spell, completely protecting the Runemaster from physical, solid projectiles. Yes, he's still vulnerable to spells, gases, liquids, etc. but, heh, if your mobs have many spellcasters whilst all being of the level of the Runemaster, I think we may disagree on the definition of "mob"!
Again, he's a Runemaster, so very likely to have, you know, a great collection of Runes with many, many, miscellaneous and different spells, especially to counter his weakness.