You can be confident of both aspects with the understanding that you are responsible for both.
The data is in a single html file that you should always be able to open in a browser, and we have multiple export methods for moving data somewhere else. The way it is stored it is only slightly less portable than plain text notes. Since you can open up the html file in a text editor and pull out your data in the worst case.
This is assuming that you are using the html version, not a hosted version like tiddlyspot. Even if you are using tiddlyspot you can just download a local backup of your wiki periodically and have that saved for the future.
And for losing data, you are responsible for your own backups. But tiddlywiki is a single small files you so can (and should!) create backup copies periodically.