Well I would expect a freehub to last longer but it depends on mileage and if it’s been looked after so the onus is on you to show it’s been serviced really. of course sometimes things break prematurely (manufacturing error or bad design), they break due to an event or they wear out early due to lack of lubrication.
Cable length was my first thought but I didn’t think about the suspension effect Speshpaul mentions so that is the first thing to check along with frame/hanger alignment.
Freehub if failing would normally slip under heavy or shock load, the latter happens more on rough ground and and your legs will jerk around as it jumps and if severe you feel like you are going to go over the bars – just like when a chain slips/snaps.
Why did you change everything and who paid for it? I would only change something like a shifter and mech if I knew it was faulty or had good reason to believe it was. Your opening post sounds more like clutching at straws. A chain can be measured and a cassette visually inspected and then go off the chain wear.