Robbie Henshaw's try shows the value of the attacking kick with a penalty

Referees have been very inconsistent lately on how they play advantage. Either the rules have changed, or the referees are making more mistakes on this. 
Sometimes it seems their is no advantage even though they have their arm up. 
I cant remember what game it was this weekend, the scrumhalf took a very quick tap and the whole opposition team was offside. He hardly made 10- metres before being tackled by players offside and the referee did not penalise the opposition, instead he awarded possession to them. 
This used to an easy way to get extra metres as the ref would push the opposition team back another 10 metres and still award a free kick. 
If someone takes a drop kick and misses, is it a certain penalty kick at goal?This would give them 2 chances to kick at goal which is an allmost certain 3 points.If they take the up and under and they compete and knock the ball on on, does the ref bring them back to the advantage spot or not. Lately it seems that making a mistake on a teams own advantage means the ref may award possession to the opposition. 

I guess I have always seen these called for team enjoying the penalty advantage....whether it's a drop, a tactical kick  or a handling mistake, if it's done instantly. But of course if the call can't be relied on, you're right, one would then tend to take the drop. I'll check it on ref.com some time.