As with any free on-line storage you have to assume that the company running the software will still be around when you need them. However, there are no guarantees. The company might go broke, BT Wholesale might pull the plug, their servers could be hacked.

If you want absolute safety for your data you should pay for a company that has servers in a least two physical locations and offer you a guarantee that your data will always be safe.

Don't ever rely on data backup in just one location if it's vital that it doesn't get lost. Get a standalone hard disk along with on-line storage and keep your data in both places.