There are really a number of factors here. Specifically,
- Style/Systems trained
- Training method (how is a given style trained. Is there contact, aliveness, technical precision, power, etc.)
- The teacher's ability to transmit the system in an effective way
- Non-modifiable attributes/anthropometerics
- Modifiable attributes (strength, conditioning, body mass, flexibility, motor control and learning, nutrition, etc.)
- Cognitive (Sport psych, drive, tactical understanding, willingness to accept discomfort/pain, reasons for training/fighting, etc.)
All of these things are going to interact to varying degrees depending on the situation.
However, the bottom line that, in situations where some of the other variables have been removed, or at least made a bit more equal (to the extent that they really ever
can be), there are some styles which consistently show the ability to perform under a high level of stress, and some which have little to no documented evidence of the same. There are some people on here that don't like to hear that, and some that don't seem to want to accept it, but it is reality nonetheless.
In addition, there are some styles which have documented evidence working when some of the variables I noted above are not equal, particularly some of the modifiable attributes and anthropometrics (aka, size) and there are styles which have little to no documented evidence of the same. Again, there are some people who don't want to hear that, but it is still the truth regardless of personal preference.
As I noted earlier, I do believe that the number of systems that will work is probably higher than often touted by those in the "aliveness" camp. For example, I don't think it is reasonable to believe that Kyokushin is the
only form of Karate that can work well against a resisting opponent with full contact. It simply happens to be the system where a charismatic leader decided to make hard contact a regular part of their training and testing, and put themselves out there for others to see doing that in ways that other forms of Karate have done less of, both against each other, and against persons from other styles with documented evidence of aliveness in their training and testing, such as MT, etc.
There are even more that probably have
aspects of what they do which will work. The actual numbers and which systems those are is harder to get a handle on, simply because the the type of training and pressure testing necessary to figure that out is either not being done or not being disseminated.
These arguments about which systems might work or might not will not be settled in any meaningful way until persons in those systems
without documented evidence of fighting ability against a resisting opponent start showing the desire and ability to pressure test their systems, are successful in doing so, and disseminate it. If they are not willing to do that, then they ought to be prepared to accept that people are doing to doubt the effectiveness of what they do. In other words, if you don't want to test your system in that way, don't get bent out of shape when people vocally doubt the effectiveness of what you do. If you want to silence the critics, you know how to do it.