What's the difference between MBP 2016 and 2017 Thunderbolt 3? I've been trying to buy some dongles for my new MBP 2017, but as I've been reading lots of reviews and product descriptions, it seems like some dongles that work with MBP 2016 does not with the 2017 version. What's going on?

Also any insight on why we can't daisy chain the dongles (e.g. Thunderbolt 3 -> Mini Displayport -> VGA/HDMI)? I have lots of Mini Displayport dongles I would still like to use.

For example look at the product description in these two links:


https://www.amazon.com/dp/B01C316EIK


  THUNDERBOLT 3 PORT COMPATIBLE with the reversible USB-C connecter that connects to new computers and tablets > with Thunderbolt 3 such as the 2016 MacBook Pro (NOT 2017 iMac or MacBook)

https://www.amazon.com/dp/B01N2ORP84


  NOTICE: the firmware of our products is updating now, please don’t connect this product to iMac 2017 and > MacBook 2017. USB TYPE C to Mini DisplayPort (Mini DP, MDP) adapter connects a computer host with a USB > Type-C port to a monitor or projector with Mini DisplayPort input, ★NOTICE★: DO NOT connect adapter(like mini > dp to DVI adapter) from other brand to USB-C to mini displayport adapter for a second signal convert, we > can't guarantee it works well.



EDIT:
Found a link with as a reference for the answer:
http://www.macworld.com/article/3200052/displays/apples-thunderbolt-display-doesnt-work-with-a-12-inch-macbooks-usb-c-port.html
You gotta love the heightened level of complexity that simplifying things down to one connector has made things.  Let's sort it out....


USB-C is the connector type.  It's the physical interface specification just as USB-A, USB-B, USB-Micro, USB-Mini are all physical connectors.  It's not a USB port, nor is it a Thunderbolt port.  
USB 3.0/3.1 is the USB specification and port type.  It sets the standard for how USB will communicate on your machine.  It's backward compatible with USB 2.0, 1.1, and 1.0
Thunderbolt 3 is the Thunderbolt specification that supplies DisplayPort, PCIe, USB 3.0 and power.  It's backward compatible with Thunderbolt 2 and 1.  Thunderbolt 3 utilizes the USB-C interface and Thunderbolt 1/2 utilize a mini DisplayPort interface connector.
Daisy Chaining - Thunderbolt can be daisy chained, while DisplayPort cannot.  Usually, displays are last in the Thunderbolt chain.


As for the Macs...


MacBook Pros from 2016 onward have 2 or 4 Thunderbolt ports that utilize USB-C interfaces
MacBooks have USB 3.1 ports with DisplayPort support that utilize USB-C interfaces.


So, when you are picking your adapter, you have to determine what port you have on your machine first.  Then you pick the physical inteface type that works for you.  

For example if you have a MacBook, you will have a USB port with DisplayPort support on a USB-C interface.  This means you can use a USB to DVI/HDMI/DisplayPort adapter or you can use a USB-C DisplayPort to DisplayPort cable.  What you can't use is a Thunderbolt cable/adapter

If you have a MacBook, you have a few more options because of the inclusion of Thunderbolt. 

As for the iMac... Thunderbolt is Thunderbolt.  It doesn't change from MacBook to iMac to PC.  A Thunderbolt adapter for the MacBook Pro will work on an iMac because, after all, an iMac is a MacBook Pro in a different form factor.