Propolis is great, but it's important to remember that there are a number of side effects too, and that propolis liquid extract or propolis tablets simply aren't for everyone.
First, as you mentioned in the article, those with any kind of bee allergies should absolutely not use propolis. It has been known to cause severe allergic reactions very similar to that of bee stings, and can be life-threatening if not treated appropriately.
Secondly, those with asthma should probably try to avoid taking propolis supplements unless they are of extremely high quality, since any impurities could possibly trigger an asthma attack.
Also, pregnant women and those who are nursing should not take propolis extract supplements, no matter where they're from. There is simply not enough research on how propolis interacts with a pregnant body, and it may be transmitted in breast milk, so those who are pregnant or nursing should not take it.
Finally, anyone who has had past allergies to black poplar or balsam of Peru should not take propolis supplements, since it can cause a similar allergic reaction.
And as always, remember that you need to consult with a doctor or nutritionist before taking propolis or any kind of herbal tea or extract, since every person's body has different nutritional and medicinal needs.