Obviously nobody is perfect, so rule that out from the start.

As for the issue if "one or two areas are out of control" that sounds kind of ominous, but maybe it's just a matter of expression.

If you mean "what if your career isn't where you want it to be and it feels out of control," or "I have some arguments with family members and I wish my relationships were a little more serene," then sure, that's fine. People are always imperfect, are hopefully always working on improving themselves, and a strong relationship, perhaps particularly, with a sub, might help a person find and focus the strength and discipline to bootstrap some unsatisfying parts of life.

If on the other hand you mean "I wish I didn't bang so much heroin but it's really beyond my control," or "I don't mean to beat my subs so badly, but I get so angry when I'm drunk" then I would say it's actively dangerous for a sub to put her(him?)self under the control of such a person.

So which parts of self control are the most important? I would say sobriety, temper, and danger of depression/self-harm top the list. Not to say someone should never get mad or sad or never have a drink, but if any of those are a big self-discipline issue for a dom (or a sub) then that should be a warning sign.

In the second tier of bad issues are, I think, chronic inability to hold down a job or have basic finances in order or an inability to maintain positive long-term relationships with friends and family. Those are pretty serious issues, but a person could live with those for the right person, at least for a while, unlike the top tier problems.

Below that are most other things. If a person eats more than they want, or would like to quit smoking but can't, or masturbates 6 times a day, or has a steady job but is unable to buckle down and study for a promotion...meh. I mean of course different things squick different people, and anyone needs to judge for themselves when entering a relationship whether a partner's foibles are something that can be borne, but I would think that these kinds of issues would be a lot less worrisome than issues of violence, addiction, or relationships.