It helps to understand that there are three different approaches to using Alexa, mostly depending on whether you’re willing to do more technical work in order to get more features.
One) Use the IFTTT “trigger” method which uses Alexa’s own IFTTT channel as the “if” and the smartthings channel and/or the harnony channel as the “that.”
Pros: extremely easy to set up, very reliable, lets you use custom phrasing like “Alexa, trigger open the front door,” works with anything (including smartthings) that has an IFTTT channel, works easily with device classes that Echo does not yet recognize like locks.
Cons: you have to use the “trigger…” Format which some people don’t like. If you want to use virtual switches you will have to create your own in smart things. And IFTTT can introduce additional lag.
Extra features: you are limited to the standard echo features except that you can easily control device classes that echo doesn’t recognize during its usual “discover devices” process.
Two) Use the official SmartThings/echo integration.
Pros: many of your smartthings devices will be listed in your echo app, which allows you to group them for direct echo voice control; you don’t have to use the “trigger” format, but instead can use “Alexa, movie night on.” (Where "movie night is the name of a group or switch.)
Cons: this breaks relatively often (since it was introduced in August, it has broken at least four times, and maybe more). Some of the breaks are relatively minor, like echo telling you that the device isn’t available but actually doing the control you requested anyway. (That one had to do with a timeout failure in the smartthings response to echo.) some have been more significant, like a class of devices that used to work with echo now not working for a while. Also, you are limited to the standard device classes that echo officially recognizes meaning you can’t control locks this way unless you use a virtual switch as a stand-in.
Extra features: none, you are limited to the standard echo features.
https://support.smartthings.com/hc/en-us/articles/205275404-Amazon-Alexa
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FAQ: Amazon Echo: The Official SmartThings Integration is here! (Initial SetUp FAQ)
- Ask Home and Ask Alexa. To use this, you set up your own AWS account and run custom code which allows you to query SmartThings and Play the answers through Alexa. This is an unofficial implementation built by community members. It is very popular among the kind of people who run their own servers. It is much too technical for many other people.
Pros: it let you query Echo for things like “which doors are open?” And adds additional two-way features
Cons: setup is complex even for those with significant technical skill; it is only as reliable as SmartThings; requires an Amazon developers account and a way to host your AWS code
Extra features: adds many significant features to echo.
You can also use both methods one and two which will let you put SmartThings – controlled devices from device classes that Echo officially recognizes into echo groups while not relying solely on SmartThings for control of third-party devices like Phillips Hues or harmony. And getting easy Echo control of the device classes that echo does not recognize like locks.
Summary
Most people seem to be either using the official method (2 above) or a combination of IFTTT and the official method (1 and 2 above), with power users who have a significant technical background using the Alexa skill method (3 above).
You will find forum discussions on all three methods, which I’m sure it can get confusing.
I myself now use a combination of one and two. I am quadriparetic, so voice control is really important to me, and I am willing to trade a couple of extra seconds for the improved reliability and simplicity of method one. But everybody’s needs are different. Other people want the absolute fastest response time, and go with method two. And people who are programmers tend to love method three which gives them a lot of features that echo doesn’t have out-of-the-box. 
I hope that helps clear up some of the confusion. 