My score has disappeared

One option is presumably to save to cloud - on musescore.com and also to save files locally.

Whether that might be a better practice than simply saving to one or the other I can't say - depends on how users feel I guess.

Saving to any form of cloud may give rise to some of the following problems:

1. Files may be temporarily unavailable - if there are problems with the internet or servers.
2. Files may be permanantly unavailable - if there are severe problems with the service provider or technology.
3. As 2, but due to operating changes, such as a change of ownership of the provider, or changes in the rules set out by the provider.
4. Files may be stolen or copied, or otherwise misused. Most providers will say this "won't" or "can't" happen, but it can - though it's probably rare enough that most people who haven't written their chart topping song yet shouldn't worry about it.
5. The price of storage might change unfavourably to the user at short notice.
6. Unless users keep their own copies as well as the ones in the cloud, the providers may make it difficult, expensive or impossible to retrieve files at some future data.
   Data stored in external clouds requires a lot of file transfers across networks - both to and from servers.
7. Data stored in clouds may be exposed to external threats because the data is transferred across wide area networks.

There are some possible advantages for users storing data in a cloud - if the problems noted above are also taken into account.

1. Files stored in a cloud may provide an extra backup if files are also stored locally.
2. Files can be accessed from multiple locations, and used on different machines quie easily.
3. As long as cloud storage is relatively cheap, it may be cheaper than buying more computers and disc or SSD storage systems for saving files locally.

Regarding MuseScore specifically, points 5-7 in the first list above are probably not too significant for musescore files, as musescore files are generally very compact. For other kinds of data which may be significantly large, and/or also significantly valuable. users should consider whether they want "their" data stored on remote servers which they have no control over.