Security wise, it is very different to Dropbox. There only you and Dropbox have the files (unless you share them), with Cubbit, it looks like “all” Cubbit users have bits of your data. If they can hack the device, there maybe a way to decrypt the data - without full knowledge of how they have implemented the encryption (code inspection, not marketing blurb), it it hard to say. But it certainly isn’t TNO.
500GB is also not much storage. Is it an off-site backup, or are the files broken up and distributed? If it is backup, performance shouldn’t be affected. But I’d look at a proper backup service, like Backblaze or Carbonite (I use Carbonite, but switched to Backblaze on my daughter’s MacBook Pro when Carbonite stopped working for 3 months and repeated re-installs didn’t help).
The energy save sounds like greenwashing. Without some more information, it is hard to assess whether their data is real or not - plus, a lot of new data centres use green energy, so if Cubbit users are stuck with coal fired power stations, for example, a data centre will still be greener.
Also, if your 4TB drive fails, there is no redundancy, locally, so you are left to re-assemble your data and you have to hope that those other Cubbits, where your data is stored, are still online and you can recover your data. If it is on a “real” cloud service, somebody is accountable for seeing to it that your data is properly stored and available, so that you can recover it when you need it.
At least if your backup cloud provider goes bust and your data won’t be available, you get advanced warning, you don’t lose a drive, go to restore your data and find half of it is “offline”.
Cubbit sounds interesting, but I don’t know whether it is fully thought through. Those are just the first couple of things I thought of, after having looked at the claims on their front page. It is a nice goal to have, but it relies on people keeping their Cubbits available.