•Is it really only that replication group for data collection only allows 2 way connection, while the other one allows more than one?
True. You can think of a Replication Group for Data Collection as a hub with a single spoke. The end points can be clusters as well as individual servers. You can create additional replication groups for data collection that point to the same hub, to make additional 'spokes,' but they will be treated as separate replication groups.
If you go through the wizard for creating a Replication Group for Data Collection, you'll see that it only gives you the option to set up 2 end points called 'Hub Server' and 'Branch Server.' Very specific use case.
•Is there any issue running only 2 servers (or disadvantages) in Multipurpose replication group?
No problem with that at all.
•If there is no big advantage to replication group for data collection, should I always choose Multipurpose replication group so I can add more servers in the future if I so desire?
You should choose Multipurpose if you want any sort of advanced replication topology. (Fully meshed, hub and spoke with many members, etc.)
From the Storage Team's blog at Microsoft:
... select the type of replication group to be created. The
‘Multipurpose replication group’ can be used to configure custom
replication topologies. This type of replication group can be used to
create replication topologies such as ‘hub and spoke’ and ‘full mesh’.
It is also possible to create a custom replication topology by first
adding a set of servers to the replication group and then configuring
custom connections between them to achieve the desired custom
replication topology.
The second type of replication group (‘Replication group for data
collection’) is a special type of replication topology and is used to
add two servers to a replication group in such a way that a hub
(destination) server can be configured to collect data from another
branch server. Let’s select ‘Multipurpose replication group’.
The 'Replication group for data collection' is really a special use case. If you want any other type of replication topology besides what is described in the second paragraph, then you want to use a Multipurpose replication group. You cannot just keep adding new members to a data collection replication group, though you can keep creating additional replication groups that all have the same hub server, to make up a kind of hub and spoke topology, if you had multiple 'branch offices.'
