You guys have already figured out that narrow knives make it harder to keep your knuckles off the board. You beat that with hand placement. Here's a view from underneath of how you have to hold the knife:
Note that the finger tips of the third, fourth and fifth finger stabilize the knife from the side, and do not curl around the handle to the bottom.
A biggish problem with using a slicer as a chef's -- at least if you use a long suji -- is flex. Obviously, longer, narrower and/or thinner knives flex more. How important is narrower? My 10" Sabatier slicer is more flexible than my 10-3/4" laser (Konosuke) gyuto. The big deal with flex is that flex causes binding. If you don't keep your knife absolutely square it will bind, and you'll either have to start your cut over or force the knife through -- at some risk to the knife of being slammed against the board when it does break free and finish the cut. If your suji isn't extremely sharp and your skills very solid, stay away from melons, thick skinned gourds, etc.
Flex is not a big deal for people who use 8" slicers; a slicer that short is more like petty anyway and is going to be pretty stiff.
Another slicer issue is that the "chin" of the knife is exposed, and if you keep the back part of the heel as sharp as I keep mine, accidents can happen. I have a bad habit of tapping the nail of my index finger against the back of my blade when I've completed one chopping task and am moving on to another. It was no big deal when my most used knives had finger guards, but now that I use the Konos so much and forget to respect the differences from the European knives with which I developed skills and habits, that exposed chin has caused me grief a couple of times. Although tapping is more idiosyncratic than not, I'm not the only problem who loses track of the slicer's different geometry and has issues with the heel/chin.
Sujis don't have enough flat to lift prepped dice into mise bowls, etc. That's a major disadvantage. They're also awkward to use for crushing garlic.
"Push cutting" means you chopping action consists of lifting the knife up and down with the edge more or less parallel to the board. A slicer does NOT facilitate push cuts. Straighter geometry does, so do shorter length and wider blades. Think about traditional asian knives: Chinese "cleavers;" chuka bocho, usubas, nakiri, and so on. Those are the knives with a built-in preference for push cutting.
Slicers tend to promote more of the tip-down, rock, then glide forward of the classic French action I call "guillotine and glide." If you're interested in the mechanics of the guillotine and glide you can read an article I posted on my blog which is already linked in my previous post in this thread. French profiles are especially well suited.
Just to continue the distinction between the three major chopping actions, "rock chopping," means keeping the tip of the blade in contact with the board (or nearly so) and rocking the blade by using the curve of the belly. Rock choppers like to keep the tip of the knife in contact with (or at least aimed) at the same part of the board, and seldom glide the knife forward. Rock chopping is facilitated by knives with a lot of belly and "rocker" along the rest of their length. 8" German profile knives are champion rock choppers.
While different geometries make certain favor certain actions by making them feel more comfortable and natural, it's important to remember that very few knives actually control your action. That's up to you. You can rock chop perfectly well with a kiritsuke, and push cut like a champ with a Wustie.
I'm not sure why this thread started. If you've determined to buy a good knife, can only afford one, and are debating between a gyuto or a suji with the idea of using it to occasionally double as the other -- buy a gyuto.