Hardness tends to offset the effects of alkalinity somewhat though it take 3.5 units of hardness (calcium, in ppm as CaCO3) to offset 1 unit of alkalinity.
The main effect of alkalinity is, as you have noted, high mash pH and this effects the beer in 2 ways. Firstly the higher the pH the more bicarbonate (the source of alkalinity) stays in the liquor. There will be 6 times as much bicarbonate in a mash at pH 6.3 as there would be in one at pH 5.3. The yeast will reduce either of those pH's but of course will have a much easier time of it if the mash was at 5.3. Whatever the actual numbers the final beer will contain more bicarb if mash pH isn't properly managed and bicarb doesn't taste very good (try some baking soda in a glass of water).
Second, high mash pH prevents the mash enzymes from doing their jobs of lysing protein and starch effectively. The result could, were the pH far enough out of line, reduced efficiency but long before that there are a plethora of other factors that result in a beer which is 'lifeless', 'flat' (in flavor regardless of carbonation) and/or 'dull'. Beginners home brews are distinguished first by stale extract and second, when they move up to all grain, by improper control of mash tun pH.
If you are brewing with highly alkaline water even though accompanied by high hardness you will at best have a beer that tastes 'minerally'. If you are not taking steps to control mash pH you will have the flat, insipid, dull character on top of that. There are, of course, lots of other ways to ruin beer besides mash pH so it is possible to have those too.
