Is there any known problem of using SIMBL?
Your question is too generic if anything. SIMBL have been the cause of past problems, specially when Snow Leopard was out; the infamous “blue screen of death” that made Windows very famous, affected OS X for a little bit on some machines that had “hacks” like SIMBL (But it was usually caused by APE if I am not mistaken) 

With that said, it’s possibly “impossible” to measure if there’s any performance degradation or “anything else”.

I’ve used SIMBL for a few years and I have no idea if I had run into issues that wouldn’t have been caused should I not have it, however, for the most part it performs ok. There are a lot of plugins already written, one of them being the excellent Total Finder by BinaryRage. I’d say that if there’s a problem with SIMBL, it would be most of the times, caused by a bad plugin that is doing “more than it should”.

With that said, I doubt Apple likes it when it’s time to diagnose a problem. ;)

When you create a Plug-in using SIMBL, there are a few recommendations that a lot of developers do not follow and that is what usually causes the problem. Most notably is the Version Checking. You are “entitled” to make sure that the version of the application you’re going to “hack”, is compatible with whatever you’re going to do. A lot of developers never did this in the past and the results were headaches for everybody.

Suppose you’re patching Safari 1.0. A software update comes and it’s now Safari 1.1 and your plug-in doesn’t work. You shouldn’t try to patch that version of Safari until you fix your plugin. Failure to perform that check caused innumerable failures and rage wars :)

You can try it and if you detect issues, removing SIMBL later is pretty simple.

But after a few long years of having it, I can tell you that it’s relatively safe; with that in mind, understand that SIMBL allows other applications to “inject” code into other Cocoa applications. A malicious user could attempt to cause damage though it. 

You should keep an eye in the SIMBL Bug List to make sure you don’t come across something that affects you and -of course- use known plug-ins and not something that came from “p0rn.com”. ;)

Now if you’re super worried about all you install and do, then stay away from it. :)