Having had 3 solidly losing days with the MSS (slightly surprised to see I have not entirely wiped out my profits over the last couple of weeks), I need to find blame. Certainly a combination of things. Some bad luck, a few mistakes, but I think there is also room for improvement in the system itself (in truth, may not be a huge part of the reason for the results of my last 4k hands). Given that I'm currently trying to play this system by the book, my leaks (other than occasional errors) are leaks in the system itself, so I thought I'd try to list them with a view to the (ambitious) idea of removing them. I'll start with very coarse detail, but hopefully will get more precise later.

1. Vulnerability to stealing
Given that the system uses a tight aggressive restealing rules, with no calls, stealing with a wide range is profitable and has the potential to make an MSS player lose more than necessary in the blinds.

2. Vulnerability to resteals
Steals from any of the 3 positions have a quite large steal/fold range compared to the steal/all-in range. As a result any steal is vulnerable to wide restealing, which is often going to be profitable even if the restealer decides to give up if the resteal fails.

3. Vulnerability to bluffing postflop
There are a number of situations in which a bluff will be profitable against the MSS, based on the betting (maybe the board too). If I was more awake, I'd be more precise.

Of course these vulnerabilities rely on a number of factors. The villain has to have a good idea of the MSS players rules. The villain has to try to exploit the MSS by increasing his 3-betting range in certain situations. And the MSS player has to not adapt to the villains exploitation by counterexploiting (if he has overdone it). But still, I feel there are some issues to be concerned about.

4. Missed EV
I think it is likely that there is another leak in the MSS due to its extreme tightness. Simply, there are profitable opportunities that are folded. But maybe not so much, as marginal hands are only marginally profitable. Perhaps more important is that the very tight ranges make a player easier to read, and a very good villain could use this reading profitably.