A couple of inches of concrete over a liner will make a big diff where you know hoofed or clawed critters are going to merrily amble about in a pond, where storm tossed branches may lance through, it will keep the liner from being damaged.
Before putting the liner in, a priority is to make sure you don't lay it on anything with sharp edges like cut roots or stones. On a small pond, it may be practical to lay old synthetic carpet remnants, or a bed of sand to make sure the weight within the liner does not puncture the liner, forcing it onto a sharp object
One downside to concrete on liner, being a slippy surface the concrete will slide on gradients when wet. Extensive forme work is used for pouring concrete where steep or vertical sides are involved.
But, as the concrete is being used to 'armour' a liner, there's no reason why laying the concrete can't be done in stages, floor, then easy sides, then tricky sides...
When making an 'armoured' pond you would get the benefit of two materials, ages durability of concrete to resist hard knocks and the ages durability of a liner which won't be vulnerable to punctures, or exposure to uv and going brittle, the main weaknesses of liner ponds...