You get to complete the asylym apparitions missions only once, after you're done you start getting the "main" ghost missions. Which are very useful to obtain PSI weapons, so that you can capture heavily armored units from which you can steal their armors (Gillman Hero, a few MIB troops) well before you can get them via research.

The problem is that the chance is only 33 % per month. With this kind of one-time mission, the probability should be at least 50 % or even more.

In my current run, I've had bad RNG and have had to wait for the mission for a year (statistical probability < 1%). But there is no point in this, because you only get to do it once (provided that you succeed). So why not make sure you actually get it soon after you have all the requirements.

For example, with 33 % probability, there's still 9 % chance you don't get the mission for 6 months running. Compare to 50 %, this would be 1,5 % and with 70 % it would be less than 0,1 %.

The same problem - getting stumped by very strange RNG for a very long time - can occur with many other missions as well (in my previous one, I had to wait for a Exalt base mission to spawn for a year). But increasing the probability of this kind for "do it once" missions would have no downside, IMHO.