Actually, some many VGA devices didn't allow changes in-between frames, as some kind of cheap hack to prevent updates of things between frames. Also, you'll find that most graphics cards will jump, and miss a few frames, if you actually change resolution in any complex way. That's because there's a phase-locked-loop-based clock synthesizer in there, generating your video timing. When you change that timing, the PLL has to re-sync, which can take on the order of many milliseconds.
The whole point of Amiga's changing resolution on the fly is kind of gone, anyway... you were trading one kind of graphics mode for another. Any graphics card still worth using these days can give you a full 24-bit color display (or better) on a couple of HDTV+ resolution monitors at once. No need to flip video modes.
Also, the only reason the Amigas didn't have the same PLL problem is that all graphics modes used the same pixel clock, or multiples thereof. That's why you couldn't independently dial in resolution and refresh rate, as you can with modern graphics cards. The never-released AAA chipset actually supported something like five different PLL inputs (the PLLs themselves were not on-chip, which made my life much harder, being the only person who ever built an AAA-chip-based system), along with other hardware to allow mixed modes,