Fishbone said:
Well, the problem I've always had was that it seems to screw the rogues over totally to have entire types of monsters be immune to sneak attack damage. I can see certain constructs, undead, and plants being immune to sneak attack but every single one is totally overkill. Especially when you want to run an undead heavy or plant heavy dungeon and you realize the key ability of the rogue will be useless in about 80 percent of the fights.
Well, one easy solution is to rule that, in most cases, things like Crippling Strike, Impeding Attack, Staggering Strike, maybe even Headshot, all work against many things that cannot suffer extra damage.
So even though the rogue cannot do extra damage to a mummy, he might still be able to reduce it's STR/DEX, or stagger it, or maybe even confuse it, etc.
Sure, some of those take extra feats, but they're well worth it if the DM allows them against Sneak-immune monsters.
Also, any rogue with any sense at all should invest in undead bane or construct bane weapons, even if they're only backup weapons. Sure, it's not as good as actually sneak attacking them, but these weapons are far more valuable to a rogue than to anyone else, because everyone else can still use their best stuff against these kinds of monsters.
In my current group, my character strongly advises these types of weapons go to our rogue first, and the other players tend to agree.
And don't overlook the Greater Truedeath Crystal from the Magic Item Compendium. Attach it to any weapon and that weapon is +1d6 vs. undead, has ghosttouch, and allows sneak attack to work against undead. Only 10,000 GP to buy, and your local adventurers' guild and your local thieves' guild probably keep several on hand (unless undead are extremely rare in your game world).
And the same book contains the Greater Demolition Crystal, which does the same thing against constructs (replace ghosttouch with adamantine). Probably a little harder to find than the Truedeath crystal, unless constructs are roaming the land in great abundance.