Madman megaturns play very, very differently depending on what the payload cards are.
For instance, if your payload is something like Storeroom, then you basically want to get 6 Madmen and 5 Storerooms ASAP. Here the overriding goal is going to be ensure that you win the splits. This requires aggressive Hermit acquisition & rapid Madman gaining. You can leave off one or two of your payload cards until the megaturn itself, provided you have enough actions (thanks to nec or something else). Most of the <= $3 cards play pretty similarly here. These are all but always times to open double Hermit and to gain/buy Hermits on T3/T4. It is pretty hard on these boards to contest Hermits without going for a big Madman turn yourself.
Life is a little dicier with your enabling cards at $4. You will have chances to get these, unless you fill up too soon on Hermits and Madmen. Killing your estates/shelters will delay the clogging, but you want to pay attention to tempo with when to buy a $4 and not gain a Madman. Copper buys are totally worth it here and should be undertaken whenever they do not endanger your odds of hitting the megaturn. If your +buy is terminal this will often setup a scenario where not contesting the Hermits is a loss, but going for the Megaturn is not going to be an auto-win without enough +action. Often it is worth it here to just play Madmen as early game enablers; try to keep one Hermit in deck while depleting that pile to stop a megaturn, but spike some early gold and provinces on a lackluster board.
For >=$5 enablers, you will likely want some Silver. Sure there are exceptions, like Artificer, but hitting $5 or more while adding dead Madmen into the deck means you want silver or better gains from your Hermit a lot more often. Bank and Goons are notoriously tricky, getting to $7 while cursing means you want to be very careful at never having too many Hermits in deck and hitting Goons with low silver & attacks is almost impossible.
In general, I find Madman megaturns are a LOT more viable than people expect. Being able to play lots of cards with 100% odds of lining cards up is pretty powerful, being able to draw & play a bunch of gains is even better. For instance, I love Hermit/Stonemason. You can really easily pile Smason and Estates from just 3 starting Smasons and one more Madman than it takes to draw your initial deck and also score some nice points by turning say a $4 card into two silvers or whacking Golds into double duchies. With a decent setup for gainers, each Madman functionally represents a whole new turn of potential.
