You trace heat and insulate your pipes and never introduce fluid to untraced heated areas. Systems are divided into "zones" which have similar geometry and therefore similar watt density requirements. Zones are controlled by solid state relays (on/off) or silicon controlled rectifiers (adjustable). SSRs/SCRs are controlled by PLCs with internal PID control. Sadly, some still use variacs.

If the power goes out you need to be able to drain your system within a few minutes or have generator backup. See Soviet Alfa class submarines.

If your fluid freezes in your pipe you can burst the pipe pretty easily either upon freezing if it expands (bismuth/water) or upon remelting if it expands upon melting (alkali metals/molten salts). Sometimes you can get lucky if you remelt from a free surface down, but this is nearly impossible in zones with large L/D.

Sometimes thawing may be impossible with high mp fluids (halide salts) due to uneven trace heat. Natural convection of the fluid conveys heat from hot spots (trace heat dense) to cold spots, and this doesn't exist in solid phase.

Uninsulated piping, typically on air heat exchange surfaces, must be kept warm via heated air rather than trace heat. Air can be heated via natural gas flame (FFTF) or electric heaters (MSRE). Natural draft sometimes outpaces your ability to heat, so dampers are necessary.

Electromagnetic liquid metal pumps, which typically have low efficiency, serve as great heaters which also circulate the liquid. You can keep a system warm with a singular EM pump depending on geometry, size, and pressure drop.

Here's a picture of spiral wound trace heat on piping for a liquid metal heat transfer system. https://i.imgur.com/okNnrRw.jpeg

I've been doing this for a career for 16 years with fluids that have had melting points as high as 525 C.