I found placing blocking volumes to outside locations very slow and cumbersome task, especially for coastal lines and cliffsides I don`t want player to fall down from.
So I started to use spline tool to make a big long splines with a simple square mesh attached to it, and set the material of the spline mesh invisible.
Is this a okay workaround or am I expected to face troubles or performance loss with this?
You can do it like that (an easy workflow), but I think it will be better when you use a blocking volume (you can modify them with the bsp editing mode), because in your case the mesh will be still rendered. 
Performance wise using geometry does not add up to much of anything but using a material even though it makes an object invisible is still a draw call where best practice is to be conservative and not be wasteful where you don’t have to be.
A better workaround would be to create any kind of blocking object you wish as mesh with collision and then select the object in place and in the detail panel under rendering uncheck “Visible”.
With this option set the object you are using for collision is culled and does not render generating a drawcall but the collision is still active.
Ah thanks! I was under the impression that it effects the collision too, but youre right.