The Mako sections in ME1 weren’t really “open world”, they were just bigger hallways in many cases. IMO none of the core Mass Effect games was particularly open world, since they were based on missions. They were all more of a “hub and spoke” design, where you travelled from one hub to another and then those hubs would have local missions you could go on.
At the end of the day, all of the ME games have pretty environments, but the pretty environments aren’t the thing that keeps me coming back to the series over a decade after its launch, the writing is.