1/2lb of lactose isn't really all that sweet. Many beers are sweeter just from the maltiness alone, and even a *slightly* liberal use of crystal malts.
But beers that are considered "too sweet", IME, are often a result of underattenuation.
Because really, if somebody is drinking a beer expecting it to be sweet - and surely, if you're drinking a style called sweet stout, you're definitely anticipating and allowing for some sweetness - a half pound of lactose should NOT leave you thinking that it's too sweet if the recipe design and the brew/fermentation process were solid.
In fact, people are often under the wrong impression about lactose due to the fact they know it is a sugar. The reality though is that lactose only has a very minimal amount of sweetness - it doesn't even come close to comparing to other sugars we are familiar with - sucrose, glucose, fructose, maltose, etc. I actually just took the liberty of doing some quick calculations, and a half pound of lactose in a 5 gallon batch will impart the same level of sweetness as adding less than 1/6th of a teaspoon of granular table sugar to a bottle of beer! That's a very low amount of sweetness - possibly even negligible - so it's hard to imagine it being solely or even mainly responsible for an overly sweet beer.
I mentioned that underattenuation is a pretty major cause of overly sweet beers, so I figured I'd point out another (related) huge misconception a lot of brewers seem to have. By underattenuation, I mean that sugars are left behind that probably SHOULD HAVE fermented, and in most circumstances, the average yeast strain would have no problem doing so. If you mash at 160° and your beer doesn't attenuate as much as the same beer mashed at 150°, that's NOT underattenuation - it's expected. It's also where the misconception lies, and for the same reason as the one regarding lactose: people thinking of sugars as all being very sweet. And because high mash temperatures leave a lot more unfermentable sugars in the wort, many people think that a higher mash temperature results in a significantly sweeter beer.
There are a surprising number of things that can make a beer sweeter, or, more often, PERCEIVED as sweeter, but contrary to popular belief, mash temperature isn't one of them. In fact, the complex sugars/dextrins that yeast are unable to ferment have little to no perceptible sweetness, and overall even make lactose look sweet as hell in comparison. But while these sugars (including moderate amounts of lactose) may not contribute noticeably to the sweetness of beer, they DO contribute in a big way to the body.