Many martial arts with a history, it turns out, are amalgamations of other fighting styles from the past as well. What usually determines when fighting methods will complement each other, or not, is these two basic factors:
Positional Philosophy
How do you enter the opponent's space, and where do you want to be in relation to them? Are you trying to push through their center and take them off balance? Are you flanking to the side, with the goal of taking their back? (Is there an expectation of multiple opponents and a need to keep changing position/direction for safety?) etc.
Power Generation
Where does the power in strikes or grappling techniques come from? Twisting from the hip, dropping weight with steps, forward/backward whipping of the back and spine, pressing off the back foot, hard to read core activations? Something else?
Two examples for consideration
Many styles of Filipino Kali/escrima tend to work well with Western boxing and Muay Thai. This happens to be because all three primarily generate their power from hip rotation and entering space from forward angles. There's no need to shift weight or footwork methods to any serious degree to switch between the techniques and the force generation being the same makes all of them work well. The context for combat more than sport, especially assuming bladed weapons are in play, means you don't see a lot of ground grappling, so not much need to focus on it.
In comparison, you can see a lot of people whose coaches have given them "Muay Thai + Wrestling" without really any thought to how to transition them. The wide stances of wrestling with grounded feet or falling with the opponent vs. the narrow stances of Muay Thai means you often see whether these folks are in "striking mode" or "grappling mode" as they will shift stance before doing each thing. (Smarter training covers this shift with a feint or attack, so the step out to wide stance or the draw in to narrow is less obvious).
