Too much sugar

What are the disadvantages of using 80% sugar in your wort compared to using DME etc?

Flavor and mouthfeel.

It will be a dry and strong alcohol taste almost wine like if you ask me. Dme will provide malt backbone and some sweetness definitely more beer like. Both will create alcohol but depends on what you want to drink. If you want beer use dme if you want some watery alcohol then use a ton of sugar.

Never tried it but it'd probably be watery the alcohol wouldn't be masked much. Sugar is completely fermentable with no other flavor compounds in it as far as I know. So Think of sugar additions as pure alcohol additions.

I once made a parti gyle brew where the second runnigs were super low, so I added dark Belgian style candi syrup that was equal to about 80% of the grist. Added hops and Belgian yeast to make about a 4% ABV Belgian Pale Ale. It had no malt flavor and did not taste like beer. Watery with a hint of alcohol and caramel.

Belgian style beers are known to contain up to 20% sugar or invert sugar, but personally I like to keep it to about 10% max when I use sugar in the grain bill.