Taken to the extreme, the concept of synergy means:
1. No magazine reviewer may apply to you. Your system may have more or less synergy than theirs.
2. No experience from members shared here may apply to you. Your system may have more or less synergy than theirs.
3. As we say in statistics, "the set" becomes infinite. If you have a choice of 5 DACs for example, the concept of synergy means that the combinations are Nx5xM, when one counts all the devices that could feed it, or sink its output.
4. No designer can know whether they have finished their job, or how good of a job they have really done. After all, per rule #3 above, they would never be in a position to have tested all permutations and as such, are releasing an unfinished product.
I could go on but you get the picture. Synergy can never be allowed to become a major factor because if we let it, the above factors come to haunt you.
My sense is that if a product is has strong weaknesses, then synergy may apply more. A low-power amp can drive an efficient speaker better than an inefficient one. But a powerful amp, would not care.
I like to think that if I test and find a great component, that its performance translates to any other place I plug it into. The alternative of chasing its perfect match is unpalatable to me and something I do not practice.
The only place where synergy plays a huge factor is the interaction of the room and speaker. In other place, I like to think the better the gear, the less synergy it needs to perform well!