1. Ultra thin watches are much more difficult to regulate and adjust. Thus, fine-tuning them to perfect levels of accuracy is more difficult.
2. While Ultra Thin watches can be reliable and accurate, they're never quite as accurate as "normal thickness" watches. For instance, you've never seen an Ultra Thin watch win top place in any Observatory Accuracy Awards. Like certain Patek Philippe and Vacheron Constantin's Chronometre Royal timepiece that were made famous because they were extremely accurate.
3. Extremely thick watches... Hard to say, but generally, high quality ultra thin watches are generally slightly less precise than high quality "normal" watches. Can't say anything about extremely thick watches. And keep in mind, we're talking about the movement here, not the case. Generally extremely thick watches have other complications on them. There are no time-only watches that I would consider extremely thick.