So as the title says, how strong do you think they are?I mean in a real fight would they offer much protection ,there swords would cut trough anything or quite the opposite on both regards?Im trying to get a sense of what would happen if they decide to invade Skyrim surface.There numbers are supposedly vast, but if they have [censored] weapons&armor that can only get you so far.

For what they use them for (mostly unarmed and unarmored travelers they surprise and capture), more than effective enough.

Against fully armored soldiers.... Their axes at least would do some damage. The things are built with just the right point to either pierce armor or damage tissue below it. Their almost obsessive use of poisons make their arrows, poor though they may be, very unpleasant and a constant threat on a battlefield.

However, it is unlikely that they would be able to pose any lasting threat against an organized, armed, and vigilant defensive force. Likely they would be hunted back to their caves, and left to rot there after the Nords find warring underground to tilt the odds a bit too solidly in the Falmer' (Falmer's? Falmeri?) favor.

The heavy falmer armor introduced with Dawnguard looks pretty damn tough if you ask me.

As tough as a chaurus's exo-skeleton; so I would say somewhere between Orcish and Ebony. Even with maximum upgraded weapons and armor, Chaurus Reapers are one of the enemies I usually think twice about facing, since it take quite a couple hits to kill them. So as Falmer Shadowmaster... That is up until the Warmonger.

But the falmer, they're blind, and mainly rely on their hearing. It would be a rather simple solution to just overwhelm them with loud sounds.

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a full scale falmer invasion of the surface seems unlikely. their weapons are probably weaker than the stormcloaks/imperials, being made out of a lesser creatures bones, probably only infused with iron.
The battlefield is a tremendously confusing, and all the noise? the falmer probably wont be able to find a single target and may wind up flailing around uselessly, unless they see like daredevil. but still, it would be extremely difficult for the falmor to successfully stage an invasion.

I could not disagree more.

In DG, they have some interesting armor options.
Even without the upgraded armor, the falmer are fearsome opponents. They go thru bandits like a chainsaw. Falmer hit very hard and can take quite a surprising amount of punishment. Their arrows and bows are weak, but they are quite dangerous at close quarters.
It is INEVITABLE that the falmer will erupt onto the surface. You can see indications in Vanilla Skyrim that they emerge at night to hunt and raid, and in Dawnguard they do significantly more than that.
When they do emerge in large numbers, it will be devastating. I suspect they will have at least a modicum of organization - after all, they are already industrious and organized enough to make potions and poisons, learn some magic, and build armor, weapons and dwellings, in addition to farming mushrooms, keeping herd animals of a sort (frost spiders and chaurus) and even keeping slaves (blackreach.)
Their most devastating tactic would be to fight at night, where they will have a distinct advantage.
It wouldn't surprise me if the falmer erupt under Whiterun and emerge from the ruined parts of markarth.

If the Thalmor ever manage to make contact and ally with them.................................