1. Cmd-Tab doesn't switch between windows, it switches between apps. This seems like not that big of a deal (you lose being able to quickly switch between windows of the same tab, you have to Cmd-` for that) until:

2. Apps can have multiple windows on multiple spaces, and Cmd-Tab preferences windows on your current space. Let's say you have apps A1 and A2, with windows A1W1 and A2W1 on space S1, and window A1W2 on space S2. Let's assume you're on window A2W1 and your goal is to get to A1W2. You have to switch to the app, then press Cmd-` to switch to A1W2!

3. Of course, this also means that the space you switched from has changed to have A1W1 on top! So if your spaces are actually monitors, this means you aren't looking at A2W1 and A1W2 together. So you finally give up and use mission control to arrange your windows, but now you're using the mouse.

The best solution I've found for this is using Hammerspoon's window switcher shortcuts, which work like Windows Alt-Tab. There's an open source app with just that functionality if you don't want to use Hammerspoon, but Hammerspoon's great so there's no reason not to go all the way.