how reliable is iCloud for long term storage?

It's a great product to store your valuable photos and videos, but note, to store them across all your devices. iCloud isn't a traditional backup cloud storage based platform. In other words, you can't remove the photos from a device once they're stored in iCloud thinking that you'll be saving space on that device because, once removed, it removes it from all devices. If you're okay with that, then sure, go ahead and use iCloud for your photos/videos. If you're looking for a traditional backup cloud storage platform, then consider such third-party applications like Dropbox, Flickr, or Google Photos (my favorite). Good luck!

iCloud is not reliable for long term data storage in my recent experience.

On Dec. 11, 2015 Apple lost some of my data (not photos) and partially locked my access to my iCloud data such that on Jan. 12, 2016 I still cannot access my iCloud data.

Therefore IMO you should have a local back up of your valuable photos and another off-site back up (Amazon, Google Photos, etc.).

iMac 5k late 2015 OS X 10.11.2, iP6s+ iOS 9.2).

evoolb I agree with your comments, i.e. no cloud storage platform is safe and should not be relied upon entirely to store one's personal information, data, etc., but rather one should consider an external source to backup their information to provider an added layer of comfort, security, and peace of mind.

With specific reference to your case you describe, I think you not having access to your data is highly unacceptable which is why perhaps I don't rely entirely on Apple's software solutions like iCloud but rather invest heavily in Google's ecosystem - Google Drive, Photos, etc.

Unfortunately I don't use either, just use Google Drive; however from what folks and friends tell me, the benefits of using Dropbox over iCloud is greater integration with third-party applications and cross-platform integration as well. I'm not sure whether prices for Dropbox are comparable or competitive to iCloud so you might want to do some research on that before. I've also heard that more apps use Dropbox built-in than iCloud.

You could run a side-by-side search on the web, e.g. Dropbox vs iCloud and you'll be provided with loads of websites details the pros and cons of each. Apparently many have gone before you with these same concerns, questions, etc.

Good luck!

What about iCloud drive? doesn't it function more like Dropbox?

It doesn't allow others to access your files like Dropbox can. Only people signed in with your Apple ID can access iCloud Drive.