Off the top of my head, 2 reasons why UD2 is not appropriate for people in your position:
1) There are much easier (read: less stringent diets) ways of losing weight when in your condition - no need to go so extreme
2) Calorie partitioning - lean people tend to partition calories more towards muscle than fat cells and this is relevant during the carb load periods on UD2 which result in major overeating.
I agree, RFL is tough. I tried it once and didn't make it past day 9 or so. That being said, the principles of RFL are sound and you can incorporate them into a less extreme diet, i.e., not as long. I'd also pick up a copy of Guide to Flexible Dieting.