The difference, if there is a chance of a random encounter at all is none. The ritual is still cast under stressful conditions. The only other possibility is that the random encounter interval might trigger a random encounter during the casting. Then what? What happens when a ritual is abandoned? Is the magus and the person being healed, if different, vulnerable during that time? I never cared for the random encounter, ever since a random encounter killed a key PC in a story I was running. It was a long time ago, but it left me struggling to come up with a different resolution and implement it while keeping the game going. I'm not saying it's impossible to do or that I didn't do it, I'm just saying PCs can do enough unpredictable stuff that having random encounters derail stories is an unnecessary burden. Also, it's a random encounter, it doesn't fit within the them of the episode, or my goals as the SG. That alone is bad. Casting a healing ritual in the middle of an episode that ends up botching is equivalent to a random encounter. Now, if I can work the random encounter or botched healing ritual into the overall story, that's great, but that doesn't always happen.
But, in the case of the other system where you roll for random encounters because of %reasons%, you have a much different setup. The random encounter would conspire to make the roll stressful, where as in a well thought out story, it stands to reason that the roll is stressful. The duration of the ritual is almost irrelevant to the circumstance creating the stressful situation. Why take two hours to do something that is in the interests of advancing the story? And if random encounters do exist AND rituals take 15 minutes per magnitude, or at least 1 hour, what happens if the interrupt the ritual half way through? Is all the vis gone, half? How does a magus disengage from a ritual? These are all unanswered questions, and it is for these reasons I think rituals need to be reworked. No one casts an Aegis moments before attack, or finishes in the nick of time, no one works healing during the attack, or if they expect an attack within the casting time.