Time intensive:
Although dissertations are due long before exams, they can take up more of your time than an exam module would do over the course of a year. Being disciplined is crucial to make sure you spend enough time both on your dissertation and other modules.
Coursework vs. exams:
If you know you are better at exams than coursework, then doing a dissertation may lower your overall mark. Students generally want to do as well as possible in their degree (a university education does have a high opportunity cost, after all). So if you know that you are better suited to exams than coursework, you may want to consider this.
Reading-intensive:
Although the precise nature of your dissertation can vary depending on if you are more of an essay-writer, a mathematician or an econometrician it’s a fair bet that you’ll need to read a fair few journal articles just to get a feel of the literature in your chosen area. This can be interesting, but is often a bit of a slog. Persevering through relevant papers is a downside of a dissertation – how much of a downside depends on you.
Doing, not thinking:
Doing a dissertation requires you to do a lot of ‘administration’: that is work that makes you act as your own Research Assistant. This especially applies if your dissertation contains a lot of empirical work, where you will need to find and clean datasets, and often merge two or more datasets. This can be time-consuming and frustrating.
Perseverance:
Although many people take pride in their completed dissertation, even dedicated academics find it tiring to concentrate on a single topic for months on end. This is less true at undergraduate level, as you’ll have exams as well, but there may come a point where you get frustrated with your dissertation.