Any technique is effecive on someone. In the dojo, you may find that you can throw newbies easily, and have much more trouble with dan grades. The same is true of Real Life (tm) - there will be people you can do particular things to and people you can't, at almost every level of aikido proficiency.
Aikido teaches two things as regards self defense. One is techniques, which you're thinking of, and the other is composed of calmness, being centered, alertness etc. These latter skills will help you much more in a fight than good technique. If you fixate on doing a technique, then if it goes wrong (which it very often might - think about how beginners in the dojo respond to their first shihonage) you'll try to force it to happen. Ultimately, you don't 'do' techniques in real live - you move, and techniques happen naturally. If they stop working halfway through, something else arises to take its place. This is also true in practice - trying to force a technique to work when its gone wrong is not good aikido (although we often need to train this way to understand why the technique went wrong).
An old teacher of mine told me that he was once mugged in a park in Glasgow by two men. One threw a punch, and my teacher automatically moved out the way (irimi) and at that point 'saw' the inherent ikkyo in the arm of the attacker. He took the arm in ikkyo, tripped up over a tree root, and got the stuffing kicked out of him and a couple of cracked ribs. However, this idea of seeing the possibilities in uke stayed with him, and much of his teaching was based on this.
Getting back to what you said - if a technique goes wrong, do something else. Use the same principles of standing in a safe place and, controlling your attacker's balance that you do in the dojo. To be honest, after 7 months aikido training almost all your techniques will suck. Don't try to use them. What can save your butt is movement. Irimi, tenkan, back step etc out of danger, then hit 'em, grab their head, or whatever.
Remember - aikido techniques are the best way to protect both yourself and your attacker. Sometimes we're just not skilled enough to be able to protect both people, and unfortunately (for them) your safety comes before theirs.