"Strong enough" is a hard question to answer.
There's two kinds of "strong" here. One is the strength against pushing straight down on the desk, and the other is pushing to the side (laterally) against the length of the legs.
Because the legs are individually attached at essentially one point, the legs act as levers when forces are applied laterally to the legs. The longer the legs, the stronger the "lever action".
Even if you got the threaded connections as tight as humanly possible, the lever action of the length of the leg would make short work of loosening up the connection--either between the leg and the mounting bracket, or the bracket and the desktop. Especially if the desktop is particle board, MDF, or a soft wood. There just isn't enough "connection" to resist racking over time.
This is why most tables/desks are built with "stretchers" or "skirting" (whatever you choose to call it) connecting the legs together at another point besides where they connect to the top surface. This makes all the legs a single "unit" which effectively becomes a "box" of sorts when attached to the tabletop (desktop) at more than one point and is a much more resilient arrangement which stands up to lateral stresses much better.
So, while IKEA and other manufacturers sell thousands of such legs, the answer is: No, it is not a good practice. The furniture built this way will fail sooner than it would if built conventionally.
Wobbly desk with IKEA tabletop and IKEA legs
Stable Legs for Desk
Stopping a standing desks from rocking