It does work but is in effect a high risk move, you would need to be quite close to begin with in order to cover the distance fast enough, and second if you do step in make sure you were throwing a technique over wise you could get caught your self with a punch or elbow while rushing in( since very view fighters throw just one kick in that sort of range, its usually the range that combos are made to work )
Also if you are stepping into kicks it may be a good idea to put up your shin guard while you step in, if you meet the kick with your shin it will cause much more damage to the other guys shin while protecting yourself.
First, I gotta say that in a thread like this one, understand that I'm generalizing. MT is a worldwide art, no doubt. The weaknesses, then, vary from fighter to fighter and camp to camp. The weaknesses I've listed are ones that I've been able to exploit in my opponents in my ring experience.
That said, I've never thought of stepping in as high risk at all. You've gotta put your licks in on the guy before he does to you. That's fighting. I see it as
opportunity.
I don't think it's very hard to learn, teach, or do. It's all about learing timing - yours, and your opponents'. It starts here with the stance(make sure you hit the "play" button):
http://www.flashmavi.com/muaythai_combat_stance.shtml
A traditionally trained MT fighter will have this type of stance, with 70% of his weight on that back leg. Before he can kick with it, he must transfer that weight to the forward leg. That's your first hint that the kick is coming, and you begin your forward movement as well.
A more westernized fighter usually stands more 50% on both feet, so one must have better timing to utilize the step in strategem against a fighter like that. However I've noticed that against western stanced MT fighters, I've had wonderful success with a lead leg thrust kick when they throw theiir rear leg. All you need to do is knock them off balance enuf to negate their kick, then you follow in forward with your own rear leg low kick.
To teach beginners the step in, we use shields to get them comfortable. They hold the shield, and try to step inside the kick. The kickers, of course, try to land the blow effectively.
The front thrust timing can be learned the same way. The shield holder initiates a rear leg kick, and the kicker tries to thrust them off.