Hey there,
Before I tackle the question in hand, I'd like to voice an opinion here, which I'd say, convoy is much safer than plunder to do. For the sake of an example, let's * to SS convoy/plunder.
While convoying, you get at least 15 coupons for sure, if you manage to complete it without problem then 30. You are able to ask for a support of stronger player, and have guaranteed rewards, plus no consequences of doing convoy.
On the other hand, while doing plunder, there is a high chance of getting 0 coupons instead of 150, for the following reasons: You need to find SS convoy to plunder, which is not always so easy, and each convoy may be protected by someone stronger than you (unless you are top of the server).
Plunder takes much more time, high risk of getting nothing at all, and on top of that, after you complete plunder, you are at risk of losing coins due to red name. Of course there are ways to go around that, but it's still an inconvenience. Just a tiny touch to fini* off, you get 4 convoys, and 3 plunders. That means, there is always a way to go unplundered, if you find a right time to do so ^^
Personally, being in top 5 of server power ranking, I find plunder to be a waste of time and too much of a risk to go with.
That being said, I apologize for this rant-ish part of my answer, what concerns a drop rate of SS missions, I agree it may sometimes be lower, but at the same time, the time frame for it is not small.
Personally I often skip SS convoy because of lack of time, and just go with coins, which are almost never getting plundered, and much easier to get.
I appreciate the suggestion to increase the droprate of SS missions though, thanks for that ^^