In my opinion effectiveness depends on one's level of ability and of course technique developed by constant practice is the only way to be effective on life and death situations.Techniques that neutralize the attacker imobilizing him are very effective and also projections or throws are very effective,depending on the situation.Throwing someone in a real fight could mean deliberately throwing him through a window,against a wall or "crash landing" him on concrete!Of course you can't practice that on a concrete...tatami or try to throw your uke through the dojo's window for obvious reasons.That doesn't make it less effective in a real fight.You can train realistically and still be inside the safety limits,that's why we learn breakfalls anyway.And once in a while you can try some surprise attacks to test your level.The attacks are real even if the knife is wooden...Let's not forget that aikido is a martial art so it should be practiced as one!
you'd have to define ability. In the sense that I believe you are speaking you are relating that to proficiency in a collection of techniques or mastery of a system. You can master a system, become very effective at it (projections, throws etc as you mention)...and yet STILL lose the fight to a guy that has no formal training or "ability" whatsoever!
This is important to understand. The individual might simply have the upper hand in the situation and NEVER give you the opportunity to do anything at all. He may simply understand the environment and control the conditions. So all that practice was for nought! You can train for years and years and STILL (most likely) lose if you fail to really understand this fact.
IMO, the chances are if you are in a fight such as defined by this thread...there will be little or no chance to throw someone or break there neck. It will simply be a fight to regain control and dominance of the situation.
In most cases by the time you get to that point you are physically and emotionally spent and have very little control over most of your own faculties much less his.
Aikido training can be a good practice to augment your training for such situations, however, in my experiences and opinion....there is a reason why most professionals and people in the business of "combat" train in a much different manner than we do in aikido in most dojos.