This mostly in regards to Assassin pre, with some mechanic pre stuff.
Assassinate is great. it's fun. it's not quite overpowered, but it's straddling the border. Except when it comes to Epic mode content.
In epic mode content, an assassin PrE is just a normal rogue. There's very little difference (that matters). As I understand it, anything with a deathward effect (which is everything in epic mode) is immune to assassinate - it does not even roll a save.
Therefore, there is no save-failure bonus dice damage dealt. Likewise, sneak-attack vorpal is useless.
I'd like to request that where assassinate is blocked by deathward (whether epic mode or not), the bonus dice from a successful save vs. assassinate is automatically applied, and the rogue is considered to have failed a sneak attack (taken out of stealth). this carries the attendant risk of pulling to much threat/enmity from the target and losing dps from losing the ability to sneak attack (or simply dying).
Similarly, for tier 3 assassins, where the sneak-vorpal effect would be successful but fails for deathward, the assassin adds her int bonus in sneak attack dice.
Consider, that with a max int build, that will be ~12d6. that is an avg or 42 bonus dmg on a failed sneak-vorpal. When discounted by 5% rate of occurrence, that is equivalent to +2 dmg per swing. Compare to slayer arrows for arcane archer: +500 dmg on vorpal (sneak or not) - discounted by 5%, it is +25 dmg per shot. This change is less than 10% of what AA get.
Also consider, this leaves sneak-immunity for undead and constructs intact (no sudden uber-portal damage).
I would also throw in as a separate matter, for mechanic-rogue, a tier-bonus passive ability to bypass construct fortification (at perhaps, 7-10% per tier, with an additional stacking bonus from wrack construct). While this probably shouldn't apply to portals, it helps to define the rogue-mechanic class and their mechanical specialties.