It's flawed because 99% of the motoring public doesn’t want an 18mpg oil burning ticking time bomb, even the RX8 forums tell a grim tale of woe.
A car is worth what people are prepared to pay for it, they can't sell a secondhand RX8 for the same as a 350z because people won't pay the price, Mazda have a good enough badge so it's not this, the RX8 has been hyped above and beyond it's abilities so it's not that either.
The RX8 is a square peg trying to fit into a round hole.
Yes, the fuel consumption is poor, but unless you do mega mileage its not really that much of an issue. Comparable cars eg the 350z only do mid 20s.
Every 5000 miles that'll cost you appox £400 more in fuel. It's not an insignifcant amount, but plenty of other cars will cost you far more to run due to more expensive, parts, labour, servicing, depreciation etc etc.
The same as above applies to the oil consumption and neither of which make the car fundamentally flawed.
The reliability issues are no doubt something to be aware of and prepared for(the same as any car!) You could also pick up a second hand 1.8t like yours and lose the engine after a few miles due to oil crystallisation, if you didn't know what to look out for.
As long as the price reflect the issues above (which second hand the Rx8 does ... and then some!) and you buy well informed, then the car can still be a good buy, or even a bargain.
But that's my point. Sure it's probably a poor choice if you were buying new, and people can't sell them 2nd hand for much because of the issues above. But for the Op, who is looking to buy an older car that has already depreciated, and who does relatively low mileage, far from being fundamentally flawed, I can see that an Rx8 is an interesting prospect at those sort of prices.