Dropbox was one of the first cloud storage vendors.
Box.com was the next most popular.
Apple had it attached to iCloud for cross device sharing
Microsoft then added it's own OneDrive
Google/Chrome/Android also includes a similar file store now
The first goal of these was to share files from mobile to Internet/laptop. After this they all added the ability to send files to other people as links etc. Most use the same levels of security in general.
Dropbox have had a couple of serious breaches (not unlike many others) and were not the most forthcoming when it happened. In the US it is normal to stay quiet for a period to allow forensic investigation but they were a good bit longer than that.https://us.norton.com/internetsecurity-e...en-exposed.html
As Dropbox (and the others mentioned) contain files they are considered a high profile target and so it is not surprising. Despite this I don't see them as any worse than the others.
Do enable the "you have had a login" as mentioned and use a strong, long unique password like all Cloudy things. If you are saving any files with personal information to it then encrypt them (MS Word has an add password option for example) individually.
In general if you are using these services to send a large personal file to someone by sharing a linking instead of a large email attachment you should password protect the individual file. If the other person should mistakenly forward the basic email with link they can get the file but still not read it. Send the password by SMS or some other means.