My opinion, FWIW, since I’ve never followed that path…
It looks like it trades stability for access.
Once assembled, the engine rests in the frame which puts pressure on the suspension system which transfers the load to the tires.
What this procedure seems to require is supporting the engine while assembling the frame around it. Then supporting the frame while reassembling the suspension.
That’s a whole lotta pieces to attach until it can stand on it’s own.
Obvious advantage: Minimal risk of scratching paint while several hundred pounds of engine and gearbox, dangling from a chain are carefully angled and re-angled into place. And have to raise the front end half-way through the process.
If you get the engine into just the right place first and support the tub properly while assembling and supporting the frame. There’ll be much fewer heart-stopping moments. Miss the pre-positioning of the engine or miscalculate temporary supports and you’ll discover brand-new curse words.
See if the published instructions tell you how to preposition the engine so the motor mounts can be bolted in. IIRC the engine drops into the left one during the traditional process…