Fix the envelope first
When the envelope is fixed, the architecture has to come to it. Coil families have to be specified at a length and bore that fit. Pulsed-power switching has to live inside the container or in a co-shipped module. Direct-conversion geometry has to recover useful electrical energy without expanding past the wall. The shielding integral has to close against the as-built dose target at the container skin.
Every subsystem competes for the same volume
An envelope-first architecture is harsher than a campus-scale architecture. There is no room for a balance-of-plant building, no room for a separate substation, no room to fix a problem by adding length. Every subsystem competes for the same volume.
What fits ships
The benefit is that what fits ships. A reactor that ships is a reactor that can be sited next to a load — a datacenter, a mine, a forward base — without civil works that take longer than the reactor itself to permit.