Max's First Law: If you ISOT something from the future into the past, the result is a wank.
Corollary to the above: The greater the technological and social advantage of the changed societies in question, the greater the wank.
Originally used in a joking context by Blue Max at the link posted above, Max's First Law is, while perhaps not an absolute truth, at least a prima facie guideline that's true in most cases. The factors behind the wank would be:
1.) Technological Advantage. A society or group from the future knows how to do more things more efficiently than people in the past do. Even an advantage of the last 20 years would be a very serious affair. Generally, a technological edge holds unless the ISOT is between a nation pre and post collapse.
2.) Foresight. Knowing what happens in the Past, at least before the butterfly effect becomes dominant, can provide critical advantages. In this regard, even an ISOT to yesterday can be a strong boon.
3.) Access to Increased Resources. Past societies have more non-reusable resources available than are available in the present. This means that more oil, coal, and groundwater can be used for a nation (or group, society, or relevant term.)
These three factors tend to make most ISOTs wanks; and in general, the further in the past the ISOT, the greater the wank. This premise is perhaps simple common sense, but despite this observation, ISOTs remain common ASB threads.
This approach is not upheld by Harry Turtledove (in, e.g., The Guns of the South) and is also strongly objected to by Thande as anachronic prejudice (or “time-ism” as the writers of 1632 call it). Thande's own story series There'll Always Be An England originated as an attempt to show how ridiculous this idea is by having the past defeat the present.