Characters absent stamina cannot use Extra Effort. Deluxe Hero's Handbook, pg 110, in paragraph continuing from prior page.
For that reason, I'd strongly recommend not going the absent stamina route. It's really unfun for a new player to be told that they don't get to use one of the game's core mechanics.
Full fortitude immunity is also a bit questionable in my eyes. Things like super-tasers and Dazzle can force fort saves that a robot should logically be susceptible to.
This has certainly been discussed before, and I've often thought that Life Support + Aging covers about 95% of the organic woes to which a robot should be immune. Then you could buy Enhanced Fortitude* to represent "Systems Robustness" in resisting other Fort-resisted Effects with appropriate Descriptors, like sensory attacks or electrocution. An interesting variant might be to have Life Support+Age, Half-Effect Fort Immunity and however many ranks of Enhanced Fort you like for a similarly-costed power that still allows a few things through at a reduced rank.
That's a pretty unusual and unprecedented way to build, though, and not as simple as just using full Fort Immune and letting Dazzles and Electrical Stuns work as a Complication. And for new players, we do want to skew to the simple.
*I'd probably also add Impervious Fort, with a good replacement 30p build as Life Support and 10 rank of Impervious Fortitude, but that would probably detract from the discussion as "Imperv suXXorz" would draw focus.
Hmmm, alternatively we could just give it Immunity (Biological effects) and put a fair price on that descriptor. Which would be either 20 or 30 points, I imagine.