It works on some devices but not others. I'd suggest that unless you go somewhere with a great return policy go to a store that has one on display and see if they'll take it out of demo mode (assuming it has one) and let you try casting to it. 

Buying a nest audio speaker imo has high chances for google cast working fine, because nest speakers are google products. Google Chromecasts normaly have no problems with casting from Deezer. So it should also be the same with nest speakers. 
At the moment Deezer has problems casting to speakers from other manufacturers using „built in chromecast“ technology and is working to fix it. 

Not necessarily true. Deezer casting to Google-branded Chromecast dongles (Gen 1 at least) and Chromecast Audio dongles doesn't work and hasn't since February.  Casting to other devices is hit or miss. 

To the OP, if this is important to you buy from someplace that has a good return policy. 

@Rick P.

For me there are and were no problems casting deezer to google speakers and chromecast devices (higher then version one). May be the first gen 1 chromecast is too old to support full casting support any longer. 
And regarding to chromecast audio (which also is very old and not longer supported) i found the following feedback of another community user (and also yourself ) having no longer issues after a deezer update.

So, „try before buy“ would be the safe way, but buying google hardware imo is a good high chance option.

And yet no other app has problems casting to them... As a long-time Deezer user I can tell you that casting has never been one of its strong points. This isn’t the first time it has stopped working, and it always takes Deezer a long time to fix issues once identified by the community.

@Rick P.

You‘re wright… absolutely. At the moment it’s not the way it should work.  Especially for all the non google speakers using „built in chromecast“… 
In my opinion deezer did some changes to their casting integration that are only compatible with newer google hardware.

May be they did the beta test for google… 

I know they are working on it together with google and it’s difficult to fix it. So imho they have to fix it soon or to go back to the way of integration using before. 
 

For the moment i just tried to give @Jannick Verstraete an advise what kind of hardware / speaker would be the lowest risk.