Several manufacturer's over the years have produces 8 foot single piece slate tables, both for commercial and home IE: furniture style tables. As was said by Trent, Valley and Diamond, but there were also Global, Delta, Brunswick tables produced in 8 foot with a single piece slate to name a few others.
The biggest drawback to most of the 8 foot single piece slate furniture tables is that they were "Budget" tables incorporating subpar or inferior materials in their design and construction. You will find a lot of these tables used cheap plywood or worse MDF in their construction.
The point however of three piece slate however is that when properly installed and leveled it should emulate a single piece of slate to a T. Obviously moving a 3 piece slate table is generally easier than that of a single piece. While finicky to setup sometimes, a 3 piece slate table offers a level of adjustability that you wont find on a single piece slate table. That adjustability allows you to "Dial" in the table to be perfectly straight and level. All of this however will depend on the skill level of the installer and how good of a job they do on the install.