I think the best way to prevent aiming offscreen when windowed is to go fullscreen, and avoiding aiming off-screen when fullscreen in a multi-monitor setup with some tradeoffs, is enough of a niche to leave to third-party addons.
To see the latency of in-game rendering, simply set a sprite to the mouse position every tick. If you move the mouse around fast you'll see the sprite is not exactly under the mouse, it's a couple of frames behind. Serious players are sensitive to such added latency and would probably prefer to see the real cursor instead. Regular gamers can always spot when a game renders its own cursor rather than using the OS one, and they usually criticise this.
Additionally pointer lock changes the way mouse input is received: the cursor no longer has an X, Y position on screen. Instead you receive delta-X and delta-Y (i.e. the change only). IIRC tracking these is not consistent with normal mouse movement, because the delta values are raw input, and the OS normally applies some acceleration/smoothing/other processing to mouse input to make cursor movements more usable. So this will either break conditions like "mouse is over object" (because there is no longer a real cursor position), or emulating them will have a weird unprocessed motion on the mouse cursor (because the OS no longer processes it in the usual way) and will throw off users since aiming the mouse accurately is more difficult with unfamiliar motion response. Neither of these issues matter for 3D games, since you look around rather than move a cursor.
So as with pretty much any technology, it's not a case of "technology X will magically fix problem Y!", it's a different set of tradeoffs which are for some purposes better, and for others, worse. The tradeoffs of pointer lock are optimised for 3D games. You can prevent the mouse leaving the window area, but it makes any visible cursor laggy, and makes aiming harder. If you want to ignore the downsides and use pointer lock anyway, third-party addons can cover it.