How can I be sure, that all of MY notes in Evernote are being synced correctly? I also use only the mac app, never checked the web interface.
This isn't a general failure of the application but it does indicate that the sync code is not robust and could result in data loss that, from the user's perspective, is catastrophic.
At best an app can build your trust over time by working correctly. But for all my critical data, I verify it is in multiple places periodically depending upon my tolerance for its loss.
The same goes for backups... you should be verifying them periodically, else don't be surprised if they fail when you need them most.
Don't don't try and put an end to disagreement, what you wrote was ridiculous.
The software you use should do the checking. A user doesn't have the time to manually check these kind of things.
"I'd rather use a service where I don't need to double-check if my data is synced or not"
Which is it? Do you care, or don't you?
Your best bet is to distribute the syncing/backup. Use the primary service for syncing, and an alternate service to backup the data the applications utilize. CrashPlan fulfills the second part of that for me.
In practice I don't have single points of lifestyle failure. I learned my lesson after I locked myself out of my Gmail account out of stupidity. I was able to regain access through a nifty little trick, the account I'd set up to recover it from I'd stopped using a long time ago, but I was able to re-register it for the sole purpose of recovering my Gmail. Now I use LastPass but I have my Gmail password memorized. If I lose access to LastPass it's not the end of the world, I can still reset all my passwords.
You want rock-solid reliability without having to engineer it yourself? You're dreaming. The only service tailor-made for your needs is the one you build.