Back to the topic of the article.
Its interesting in my personal opinion, but its also a bit misleading to read.
My comment/thoughts on it:
A link to unsubscribe/leave a group: Where should this link lead to? (dc has own no servers) the only option I could think of would be a mailto link that lets you sent a predefined message that dc clients could recognize and remove you from the group. But I’m not sure whether mailto links work in all email clients. (The point is that the most people that see a link probably think it leads to a website)
That point about spammers is not a real point: even plain Thunderbird or a simple php script is better suited for spamming.
The next funny point is “The privacy is broken, because you can see email addresses” - For email you need email addresses, it’s not possible to hide email addresses when you have a normal group (for mailing lists and channels its another topic the latter one would use bcc, but there you can’t write in the group. Also both are not implemented at time of writing, for progress on those see New feature: Group Types)
Also you can get an anonymous email addresses dedicated only for your deltachat account, so nobody can spam you on your main email email-address.
I continue in german:
“Außerdem fehlte in den Mails die gesetzlich vorgeschriebene Abmeldemöglichkeit (Opt-out) bei dem Gruppenversand” - Soweit ich weiss gilt das nur fuer newsletter und mailinglisten.
Naja sehen wir dass ganze “Datenschleuder” gerede mal als hilfe schrei nach den newsletter features 
Fazit:
Der Artikel ist missverstaendlich, weil suggeriert Gruppen seien unsicher und bedenklich, dabei meint er nur die Gruppen, die als Newsletter verwendet werden.