That's a bit of an overstatement. It's very slow and, like any ClickOnce application, is a pain to get working under Wine.
Searching around there were apparently better tools, but Curse shut down their access. Imagine if GitHub were to stuff their desktop client full of ads and then require you to use that instead of the Git CLI.
And if they do merge it with Curse Voice (as they claim to plan to) then I expect it will stop working entirely...
For syncing/backups you can just symlink Interface/Addons and WTF to somewhere in Dropbox/Owncloud, no need for a specific application (and subscription) for that.
Full disclosure: A few years ago I published an add-on on Curse and apparently I still have free premium ~80% of the year after that. I still use their client because they're the only game in town these days, sadly.
Are you able to say why you think Curse is terrible?