Is there a problem using electrolytic (polarized) capacitor as coupling capacitor in SEPIC converter?
The Electrolytic cap does see a DC voltage in your converter. So it will not blow up due to reverse volts. Ripple current is an issue ,Film and ceramic caps are better in this respect. Check your ripple current ratings for your anticipated product life at your expected operating temp. If you want high power high reliability at high temperature then you will struggle to find a suitable electro. The coupled coil does have some leakage inductance so there will be voltage spikes on the MOSFET Drain which will get worse at high current. Consider a snubber if there are EMC concerns.