> According to my physiotherapist, cycling (really any movement) is great but does not necessarily help with back problems and in some cases even make it worse (probably depends on bike, saddle, position etc.).

If you stay in one position on the bike, and bike for a long time (hour+), it's probably worse for your back. But if you switch positions, you can find some nice stretches sometimes. Depending on the terrain, you might have to specifically remember to stretch your back while you're on the bike.