I have this fear as well. I even took my paranoia a step further after finding some research that there's a genetic factor to how well bariatric surgery works (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3644642/). I actually did a DNA test through AncestryDNA and uploaded the results to Promethease so that I could see if I have one of the "bad" genes and I do. In the study I linked to it correlated with losing about %5 percent less body weight. For me that would be me being an extra 20 lbs heavier and that's just one gene. So if I have 200 lbs to lose and I can expect to lose, on average, 70-75% of my excess body weight with RNY, that's 140-150 lbs. With that gene I have I might have to adjust that to 120-130. At 120 lbs lost I wouldn't really be where I want to be, but I'd be MUCH better off than I have been.
In all honesty, I sort of regret checking this gene. At the end of the day I don't think it makes sense to let 20 lbs influence my decision and it's just one more thing to discourage me. In case you're wondering, something like 95-96% of people have the "good" gene. I'm one of the "lucky" few with one of the bad variants (there's another variant that is even worse, but it is VERY rare). It's also possible I have other genes that will influence my success, for the good or the bad. I think the science will get better over time, but right now I don't think there's a great way to know how you're going to do without just taking the leap.
I haven't officially made a decision, but I'm leaning pretty heavily toward RNY. I may never be thin, but I think I can be much less fat and that has enormous health benefits, even if I never come close to my ideal weight. We shouldn't let perfect be the enemy of good.