I think working in one area is good if you know that is the area you definitely want to work in and when you don't want to explore other areas of law to find out what is right for you. I think many candidates applying to TCs don't know what area of law they want to work in, and so trying different areas out to then work out what might be best works for many. But other candidates, especially those with experience as paralegals may be more sure of the area of law they want to qualify into it.
I think such opportunities are opening up given there is no longer the restrictions on seeing three distinct areas of law like there were with the LPC/period of recognised training route. It is likely this will eventually be seen across a number of different types of law firms, not just through training contracts but also through people qualifying via QWE gained from other legal roles. There have been some specialist TCs already, like the one that
@Amma Usman has mentioned. Firms like
BCLP have specialist Real Estate TCs too. I don't know the full list but its definitely something to consider if you think it would be better suited to your career aspirations and goals.