What Robin S said.
Security
How does the widget communicate with the third-party server? Is that something in your control? Are there risks if the communication was intercepted?
User-Friendliness
As a general rule, I would never put any sort of code in front of editors, unless I have to. Edit 10 or 15 pages and one widget starts to look like another, we miss or add an extra apostrophe here and there, and soon, it gets a bit messy and our code trips.
In addition, the editors will probably be copying and pasting and that's somewhat tedious. Other than that, it doesn't look too friendly . OK, we can mitigate this by asking the client to enter something like this in the text/textarea field
type:class_lists,partner:mb,id:123456WB,version:0.1
OK, a bit of progress, but that's still a bit cryptic for some clients. The ideal situation would be to create one input per editable-value. That would mean 1 field per editable-value. Maybe an overkill (to some). Better yet, I would create a custom field that will hold all those n number of inputs for n number of editable-values. The advantages of this are:
It is user-friendly
It is easy to sanitize and validate each individual input
If needed, we can query each of those fields (e.g. in case we have lots of pages and need to find those that contain xxxx in their ID in order to edit them)
Etc...
Below is an example custom widget field as per above. It is not very difficult (if you are a developer) to create one based on the Events Fieldtype.
Just my 2p. Something to consider.