I was hoping we wouldn't have to go into this, but I'll name a few which all relate to the mouse being drawn by the game and not by the actual OS.
Playing on larger maps on an older computer (sometimes I play this on my 'retro' machine); the mouse will feel
extremely sluggish.
This is due to the game not keeping a consistent frame rate and the in-game mouse cursor not being able to keep up with the actual mouse position.
Playing on smaller maps on a newer computer; the mouse renders so fast it becomes invisible when moving around.
No Vertical Sync or Triple buffering option in your GPU settings will solve this.
Playing on very high resolutions; the mouse will be very small, to the point where it's almost invisible.
Changing the interface size will also scale the mouse cursor.
I am very happy with the size of the interface when using a different font as well as double sized interface.
But the mouse looks atrocious.
Example:
One of the key rules to making a game feel responsive is to
never interfere with the controls in any way.
Which means that the mouse cursor should act exactly like the player is used to outside of the game.
Never change the speed of the mouse, never change the (double)/click speed, never add or change the acceleration of the mouse etc.
A Mac mouse cursor should act like it does on every Mac computer, a Windows mouse cursor should act like it does on Windows.
Any change -- big or small -- means the player has to adjust to the new changes, making things feel unfamiliar and also less responsive.
OpenTTD doesn't break all of these rules of course, but in general the mouse is a bit unpolished and doesn't act like you'd expect it to, which makes it feel "off".
A hardware/OS cursor is what we're used to, so a "simple" solution would be to just use that instead of trying to mimic the OS' mouse cursor.