This is a good discussion to have.
Tresorit seems to be the most mature end-to-end encrypted cloud storage provider available. In terms of quality on both a personal and business/enterprise level, it is on par with and a serious alternative to Google Drive, Dropbox and OneDrive who do not offer E2EE, as a secure and private file storage and collaboration platform. They offer clients for all platforms and also have email addons/plugins.
They have strong regulatory compliance when it comes to GDPR, CCPA, HIPAA etc. Security seems very good and certified after having undergone an independent third-party audit.
They also offer Tresorit Send as an E2EE file sharing service, which is available as a browser extension as well.
As for its closed source model, needless to say, it’s not an accurate indicator of privacy and security. A great example of this is MEGA which claims to be E2EE and has fully open source clients, yet its encryption was found out to be so weak, broken and full of holes that user data could be easily accessed and it might as well have been stored unencrypted. For those with high risk threat models that are unwilling to trust the provider, using additional client-side encryption tools is always a recommended option before uploading.
Usually we have no reason to recommend a closed source product when a much better, more secure and more practical open source alternative exists, but the cloud storage options are lacking, and while Proton Drive is great and has extremely promising potential to be the best-in-class, it still needs years to fully mature, so this seems worth examining and considering.