Where? As far as I know, all the Deutsche Bahn Tickets for regional traffic use the Deutschlandtarif, which is accepted in all "normal" (excluding heritage trains and the likes) regional trains in all of Germany.
The example I had in mind was for an S-Bahn in Karlsruhe (operated by AVG, using the S-Bahn trademark under license, but otherwise unaffiliated with Deutsche Bahn) which was considered a tram, not a regular regional train while it traveled within city limits. This meant that DB tickets weren't necessarily valid, and AVG claimed as much. I expect AVG recognizes the bw tarif under which DB sells train tickets, so this probably is no longer an issue.
Is the Metronom (Hamburg-Bremen) covered by the 49€ ticket, too?
A quick Google seems to suggest that that's the case, and their website is advertising ways to buy the Deutschlandticket pretty clearly.