If the visibility is good, I rely on AIS to help me monitor the predicted closest point of approach.

I have used MARPA on our 2002 vintage Raymarine radar and chart plotter with good success. I have rarely needed to track more than one vessel. I think my unit would have trouble tracking a scad of them.

When signals get weak, sometimes the MARPA tracking will tell me that the target is making 100 knots or more. That's about the time it vanishes from radar. In rough seas, accuracy deteriorates. MARPA relies on the relative target bearing combined with my boat's fluxgate compass and (I think) rate sensing gyro built into the autopilot. When motion gets corkscrewy, then the calculation of target direction and derived target course also get less reliable. That in turn throws off the calculation of closest point of approach. I have been surprised that MARPA works quite well on our boat overall.