From what little IÂ’ve seen of Systema practitioners the ones who have studied other arts all seem to be much better combatively than the ones who have only or mostly studied Systema.
It's funny, I haven't found that to be true. Systema practitioners who have studied other martial arts, particularly if they achieved a high level in their previous training, continue to do the other martial arts instead of learning Systema. That makes sense, as if you spend many years learning to block, it is not going to be easy to un-learn all that and yield instead of block. Some styles that are very compatible with Systema, like Aikido or dancing (OK, it's not a martial art, but still a great background for learning Systema), seem to give a definite advantage to learning Systema. Other styles like karate and kickboxing seem to be more of a hindrance until the practitioner can empty the proverbial cup and accept new concepts.
Of course, as has oft been pointed out, previous martial arts training in any style gives a good background in biomechanics that is helpful to Systema as it would be to any martial style. It's just my personal experience that the psychological difficulty of letting go of previous training that very contradictory to Systema principle is harder than starting from scratch.
Combatively, anyone with martial arts training is probably better off than someone without it. However, I have observed that students with no previous training are usually more skilled at Systema after, say, four months, than students with previous martial arts training, who are often still practicing their other style in Systema class even after four months. It sounds crazy but I have seen students with only 6 monthos or a year of training easily handle students who have had years of training in another martial arts. Hard to believe until you see it yourself.
That's just my perspective from working with hundreds of different students in the last three years or so. In the interest of full disclosure, I myself had no previous martial training before undertaking Systema so that might slant my perspective on this question.
Best,
Rachel