I always think the Cavalry triangle only made sense in Nasuverse context.
Casters usually prioritize making workshops and fortifying them, making it super hard to get tactical advantage over them, especially if they’re playing the waiting game and hole up in their workshop until the last Servant is left alone and beaten up. They are the natural counter to Assassins that rely on recon without having the raw power to fight without tactical advantages.
Assassins with a competent master usually lay low and gather intel of all the servants in the war, and once the battle is in full swing, they’d begin sniping other masters. Riders are especially weak to this due to their high mobility and reliance on short NP bursts due to their lower stats than the Knight classes. Because Riders will more likely move around and do hit and run tactics on other Servants, they would need to either bring their Master with them, or leave them behind in their home base. If the Master is with the Rider, then they’d be prime target for a trap or an ambush. If the Master is left alone, then the Assassin can, well, assassinate.
Riders are statistically the strongest of the Cavalry class, owing to the fact that they are actually a combat personnel by design. However, they’re still an inch short from the Knight classes, so they still need leverage to beat them in straight combat. This is where their NP comes in. While the Knight classes NPs are “big decisive attacks”, Riders have “mounts they can throttle on demand” that focus on doing repeated chip damage (neither Iskandar’s Gordius Wheel nor Medusa’s Bellerophon are decisive attacks. Shinji was just stupid and banked on the idea that Shirou as a Master is too weak to let Saber use her NP.) Owing to this, a Rider’s optimal strategy is hit and run. Hit and run, and a siege. A well prepared Rider can run and resupply, then come back to harass a Caster without allowing the Caster the time to restock or run away, provided they can brute force the hard defenses and disable/sabotage the soft defenses/supply line.
“B-but Iskandar didn’t act like that!”
You might not want to admit it, but the crux of Iskandar and Waver’s tactic is flying around the city to find battles they can crash, and then making a timely retreat when appropriate. I do think that Iskandar’s Ionioi Hetairoi is not a typical Rider tool because it’s a “big decisive attack”, but still Iskandar didn’t pick any fight other than the one where Gil executed him. The one where he beat HF Hassan was a fluke where they reveal their entire 100 bodies for no discernible reason.