I think it’s really common and easy, without very careful and thorough testing of a kind few people actually do, to lose track of all the times that an automated trailing stop moves in the direction of your trade just enough to stop you out on some kind of a correction or reversal before the price then takes off again in the direction of your entry. Big winners can become small winners, this way. Because they’re wins, it’s easy instinctively to look at that as a “success” and not realise that it’s actually an expensive error.
Another problem is that with some platforms (for example MT5) you actually have to stay connected to the web for a trailing stop to work at all.
I think our mutual friend summarised it well, here:-