There’s nothing wrong with stranded wire, it just means if it’s carrying voltage and a few strands snip or break (leading to greater than 15% voltage loss) it can cause issues. Solid is simply an all or nothing kind of wire.

The only thing I use solid core for is fire alarm systems (where all or nothing is crucial), certain audio equipments, and anything nearing high voltage which a thermostat generally reaches. If it does take 120v+ then wow what why