A welded diff is a diff that doesn't work at all.

The whole point of a diff is to let the wheels turn at different speeds when you corner. An LSD prevents this, but only when you want it to, because it reduces power to the wheel that's spinning and increases power to the wheel that is has grip.

An open diff will ALWAYS give equal torque to both wheels, so when one loses grip it will spin faster while the tire that does have grip will only have 50% of the engine's torque available. And LSD locks both wheels together so even when one wheel loses traction, the torque that would have been lost to spinning gets used by the wheel with traction.

OTOH, a welded diff always has the wheels move at the same speed, which is good for drifting, but makes regular driving very hard around corners because of drag from the outside tire. Clutch-type LSD's will have a similar problem when they have very high initial torque.

FWIW, viscous LSDs are f*cking useless. Mechanical is the only way to go, either helical (Torsen) or clutch type. But for motorsports only the clutch (or cone) type is really ideal. This is true of both drifting, autocross, and track racing.

The downside besides the upgrade cost itself is the noise, street driving, and maintenance cost.