SPICE uses modified nodal analysis (MNA), which leads to differential algebraic equations (DAEs). This is very well-suited to detailed semi-conductor models (a bunch of convergence helpers are used behind the scenes to make the simulation of extreme exponential transients possible).

PLECS, on the other hand, uses ideal switches (on/off) for MOSFETs, diodes, etc. This is, in a way, the "most extreme" form of nonlinearity. But by leaning into it, we can solve more efficiently the resulting circuit equations using far fewer degrees of freedom. PLECS uses a piecewise state-space fomulation (ODEs). A simplified approach like that is basically essential for high-frequency power electronics at system-level if you don't want to wait all day for your waveforms. But obviously the trade-off is that you can't see the switching transients, because they are instantaneous.

To get both type of analyses (zoomed in device-level v.s. multi-physics system-level), we had to build the complex schematics in two different softwares (PLECS and SPICE). This took a long time. It's also quite error prone, especially when complicated initialization or post-processing scripts are involved. With PLECS Spice, we solved our problem. We can build one schematic in PLECS and "spice-ify" the bits we care about within it using a configurable subsystem.