I can't see why it would be a problem. Ours has been running for years, different cheap brand, no issues. I don't run it 24/7, I have it on a cheap digital timer so it runs when I want it to - during cooler times of the day in summer, warmer times of the day in winter, plus a bit in the evening / night for general ventilation. No moisture issues here as we remove at source, double glazed so no wet windows, etc.

thanks @timmmay

So when you use it, is it always at fastest fanspeed?

I'm inclined to think it's fine as well given that 1) the manufacturer allows consumer to choose that setting (they could lock out that speed) and 2) manufacturer's manual doesn't say "don't set speed to 3".

However maybe 1) and 2) are because it's in manufacturer's interest to accelerate wear and tear of parts, hence the question.