VAC is one of the most, if not the most, complex anti-cheat solution out there. I've spent a lot of time around pretty much all anti-cheats because I like breaking things. Anti-cheat software, at a high level, works similarly to anti-virus software. I won't go into detail about why I think it's so cool in this post, but I'll explain the reasons behind why it acts like it's "garbage". The issue is both technical and political:
Technically, you have to ensure that your anti-cheat is sound. You're defending not only the game, but also yourself. Similar to viruses going after the antivirus to conceal itself, if a cheating program can just disable you it will. That is by far and away the easiest way to bypass anti-cheat. Earlier versions of VAC actually allowed this to happen.
Additionally, you have to ensure that you're not wasting everyone's time by flagging things that aren't cheating programs, therefore you have to develop a tried and tested system for immediate flagging, and a system for "further research". This is what VAC has. The tried-and-tested method will ban you automatically, without any human intervention whatsoever. LMAOBOX free will have you flagged immediately and at last check I believe the time-to-ban is approximately three hours, as everybody knows this is VAC detected. This detection is done based on what is generally assumed to be 'hashes' of the binaries available, and their known patterns of behaviour.
The "further research" piece is based off heuristics. If VAC thinks that some software is doing strange things to the game, it will make a note of what it does, and send that data back to the anti-cheat team within Valve (yes, they have an anti-cheat team).
On top of all this, you also have to ensure that it is reliable, and does not impact the performance of the game. You can't have the game crash out due invalid memory accesses and other such things, that's bad for business. You have to balance all this against man hours available. I know that VAC has some issues within Linux and OS X that can't be remedied easily, for instance. Given the amount of players - and the amount of cheats - for those platforms, I doubt they'll be fixed any time soon.
Politically, you have to ensure that the anti-cheat has a(n almost) 100% hit rate. If you're reporting false positives day in, day out, legitimate cheaters will slip through the cracks, getting unbanned, and causing issues in the above mentioned detection processes. This is why VAC, and Valve by extension, are adamant that a VAC ban was a definite cheating hit. They stand by the mentality that it's better to let the few cheaters slip by instead of accidentally banning a legitimate player. LMAOBOX premium has a few "features" that make it really difficult to detect reliably and consistently. It's not a miracle of software engineering, it's just a fact of being just enough subterfuge to slip by because of Valve's restrained policies.