The best time to semi-bluff bet/raise is in position and against few, preferably 1 opponent. The value of the semi bluff comes from the combined fold equity + the equity of your draw. Example, a flush draw on the flop with 2 cards to come is roughly 35% to hit.
If your opponent folds 20% of the time to your flop bet/raise your combined equity is now 55% which makes it a profitable play. The Hypothetical fold equity decreases as the number of opponents, stacks and pot size increases.
Being in position is important because you need to see the turn and the river to have 35% equity, if it much harder to get a free card OOP when you don't hit on the turn, or even IP vs multiple villains, now you have to decide, with less equity if you are going to double barrel and put more money in behind.
That compounds the situation when the semi-bluff does not work they still want to shovel their chips in on the next street, the villain(s) said we don't believe you by calling and you respond with FU I missed and am still shoving and get called and are in bad shape.
In general, especially in multi-way pots, when opponents let you draw cheap, and at a good price, you should. You want to keep your Implied odds as high as possible.
Here are 2 good threads to read from LLSNL that show how betting a draw at the wrong time turns a +EV situation into a -EV situations.
This shows how Check/Raising a draw as a semi bluff OOP, even against 2 opponents, can be bad:
http://forumserver.twoplustwo.com/17...s-oop-1499186/
This thread shows how building a pot even IP with a semi bluff can lead to tough spots when fold equity is low
Hero here made a not so great raise preflop, fired on the flop with a good draw and is now in a tough spot on the turn once being called, he ships when villain is basically never behind and never folding.
http://forumserver.twoplustwo.com/17...-turn-1499641/
read DGIHarris's post in this thread.