Try this and post the output in a reply:

aptitude search vivaldi

Has anyone had better luck with Brave???

Yeah at least on Daedalus, running it right now, works fine after you do some initial work on settings. Ad-blocking is pretty good.

Main thing I like about it, is that it's not Firefox. Second thing I like about it is that it's not Firefox. No ridiculous scrollbar nonsense, that's another plus.

What's wrong with it?

It's proprietary, so who really knows?
"Negative feelings" about closed-source browsers and packages that are not compliant with Debian free-software guidelines or packaging policy, on the discussion board for a Debian-based FOSS distribution, are not particularly surprising.

That's really not a very high bar to pass, typhoid is "no worse" than lung cancer, but I for one don't want either.
Brave and the like are at least open-source, so any dubious "features" are readily inspect-able.

As for Vivaldi, their website claims that after installing the the .deb package "our Linux update repositories will be configured automatically for you to receive updates".
Personally I doubt this "Users are stupid so we don't give technical details, just click button and we'll handle everything" corpo-speak bullshit, but I'm not about to install the thing to find out.

I can vouch for this when I used Vivaldi a while ago. After installing it, it was showing in Synaptic and updates came through it (Synaptic / apt). It has been a while since I've used it, I'm not quite sure of all the negative feelings being said about it here. What's wrong with it? It's no worse than Chrome, is it? Of course that's not saying much anyway.

Anybody know how to get rid of?

apt purge vivaldi-stable

...probably. Assuming the package didn't do anything else idiotic in postinst. It doesn't appear to, but I didn't look very hard.

Is a janky PoS at the best of times (ditto packagekit in general). Use apt at the CLI, or if you must have a GUI, synaptic.

https://wiki.debian.org/WebBrowsers

Has anyone had better luck with Brave???

It puts the browser in the opt directory and nowhere is there to be found a gpg key. They install a cron job file which when trying to look at has nothing in it so it is a bit of a mystery what it does.

Everything is inline in the package postinst script, the repository configuration and key signature (base64, for reasons) are piped into the config files... Because of course that's how they do it - it's the best way to fuck around with the system (making sure their browser is the very highest priority in alternatives for example) and "support" multiple distros with a single .deb.
The crontab file is a symlink to /opt/vivaldi/cron/vivaldi, which is a copy-paste from the postinst and apparently rewrites the apt configuration and keys every day... Again, reasons, presumably.

Proprietary vendors are all the same when it comes to packaging, total disregard for established standards. Everything that can be an executable script is, windows lets vendors screw around however they like and execute whatever they want so that's how it must be done everywhere.

As for Vivaldi, their website claims that after installing the the .deb package "our Linux update repositories will be configured automatically for you to receive updates".
Personally I doubt this "Users are stupid so we don't give technical details, just click button and we'll handle everything" corpo-speak bullshit, but I'm not about to install the thing to find out.

I did not install the piece of trash but did download and extract the package and its data payload. It puts the browser in the opt directory and nowhere is there to be found a gpg key. They install a cron job file which when trying to look at has nothing in it so it is a bit of a mystery what it does. I tried their junk once many years ago and gave up once I discovered they were more interested in controlling their "smart" lights from the browser than actually fixing the bugs I reported. My reward for that effort was constant spam from the scumbags on the email that reported the bugs such that I had to create a special filter for anything coming from them going straight to /dev/null. That is the type of people you deal with when using it.

For brave, you might want to try extrepo.
As for Vivaldi, their website claims that after installing the the .deb package "our Linux update repositories will be configured automatically for you to receive updates".
Personally I doubt this "Users are stupid so we don't give technical details, just click button and we'll handle everything" corpo-speak bullshit, but I'm not about to install the thing to find out.
AFAICT, this is pretty much what should be on that page WRT repo setup and key import. The security implications of using /etc/apt/trusted.gpg.d as in this example are explained in the wiki above.

I don't use either of these packages myself, so YMMV.  Frankly why people need multiple "privacy" browsers proprietary and/or crypto nonsense packing chrome forks from third-party sources rather than just configuring the tested offerings in the stable repos to their liking is beyond me, but whatever. vOv.

Also, as always, DonkBreakDe[bi|vu]an. Note comment on "other repositories created to distribute single applications".