That's still a huge disadvantage, as it piles up more checks to the list. And with lower speed, more Scarfmons can take you on. With Slash, you have to choose between cripplingly low speed and some wallbreaking power or mediocre speed with no wallbreaking power. >_> If it really held a candle to Doryuuzu (which it doesn't), more people would be using it. I'd certainly face Slash before Dory if it means I can get a wall or bulky sweeper in on him and KO.
EDIT:
This statement is also a load of bull btw. The problem is that when considering Slash is 35 base Att weaker than Doryuuzu, he has way more checks and counters than Doryuuzu does.
For example, consider X-Scizzor on a defensive Celebi: 69.3% - 82.2%
Now let's try it with Adamant LO Doryuuzu: 95% - 111.9% (on top of that, defensive Celebi can't KO back with a Grass move).
Most Doryuuzu run Jolly Balloon. And if you remove everything that can counter or check something and get a chance to set it up, of course it sweeps.
And yes, some scarfers can take on sandslash, but they are few and far between. Base 105 speed is already very good, most pokes above that usually don't go scarfed.
The main reasons people use Doryuuzu over sandslash is becuase:
1) Sandslash loses 1v1 to balloon Doryuuzu- the same reason most people run jolly balloon doryuuzu
2) Hype over hi s135 base attack, new pokemon, etc. Sandslash kinda fell by the wayside.
I've used both, and I found I preferred Sandslash.
Also, a list of pokes who can check Dory but not sandslash:
Roobushin
Infernape
Technitop
Anything else with mach punch and decent attack, and stab
Lucario
Skarmory (sandslash has possible 2hko with life orb and jolly after SD and SR)
Sandslash but not Dory:
Celebi
Lati@s
Scarfers 105 base speed+
Probably Tangrowth, and other defensive grass types (Celebi itself is rare this gen, let alone other grassers).
You're also overlooking the fact that while Doryuuzu's stab move is stronger, Sandslash's rock move is stronger.
And about the Balloon vs LO Dory: Dory's ground weak is quite crippling, causing him to lose to hippowdon, Other doryuuzu, especially balloon variants, Ditto (if DW), and others. Sandslash, however, does not have ground or fighting weaknesses and thus is free to run LO.