Multiple Database in One Server Problems Our company has 5 different databases. None of the multiple databases relate or utilize cross database joins.
We have ample resourcing and budget.

Our company is debating between:


5 databases on 1 server instance, 
or 5 databases on 5 different servers.


Question: What are some specific problems, obstacle issues which can occur, if we place everything on 1 single server? My background is not DBA, so would like to learn examples I can discuss with team.

Background: 

The argument for 1 server instance at company: We have "Resource governor" and "AlwaysOn Availability Groups" with Failover clustering.
Resource governor will handle Memory, Cpu, io workloads between databases. So along with AlwaysOn DR, it can handle runaway queries, bad design, memory issues.

Databases are 100GB each, critical company information, 50 million rows minimum

We process 300 transactions/sec

RTO = 10 min
The answer is classic -"It depends !" 

Sit down with your stake holders and present then the pros and cons of the 2 options. Being in the business of client hosting, I have faced this situation and below is my view point :


  None of the multiple databases relate or utilize cross database joins. We have ample resourcing and budget.


This means that the dbs are decoupled or independent. I would say, club the dbs depending on the criticality and uptime SLAs and then distribute them between 2-3 servers (VM would be another option as well). This way, you dont put all your eggs in the same basket.

Why ?


Putting all 5 dbs on one server will be a single point of failure. An issue with a single DB puts all your other DBs at risk. Isolate your risk as much as possible but that will cost you additional licensing and maintenance overhead which you have by saying - ample resourcing and budget.
Even if you have Resource Governor enabled, it does not govern buffer pool. It only governs "working memory of a query e.g. sort, hash etc". In 2014 and up, you can limit IOPS using Resource Governor.
You have one solution for a given server hosting all your dbs to do backups - full and log, integrity checks, index maintenance. If there is an issue with your server and that job fails midway, other dbs will be impacted except you have inbuilt mechanism to detect and fix the issue and continue with the rest.
An external issue with OS affects all your estate being in one place.
Even with AGs, you have a fix amount of worker threads and they get shared with all your dbs on a single server. Having dbs spread out depending on the criticality will alleviate that problem.
Tempdb is one per server. So imagine all your dbs being on the same server might be a bottleneck.
Problem isolation will be difficult if you have one server serving all your dbs.


I have written a sort of similar answer Which is better: one database per application, or just one database?