All 3 Megalith cards are great, custom even, but Megalith as a deck will never compete in any serious capacity because it suffers from the same problems Ursarctics do.
1. It requires you to open very specific hands to be able to do anything.
2. It's too resource intensive early on. As such if you get interrupted you've just wasted like 3-4 card and achieved nothing.
And just like Ursarctic it does absolutely explode in the card advantage it can generate later on in the combo, often ending on 6+ cards in hand and a stacked field but early on it just takes too many fucking resources to do ANYTHING because of the nature of the decks playstyle and how the cards are designed. You need 1 of the higher-level megaliths to act as the ritual spell (discards itself to activate), 1 in hand to summon and tribute fodder. That's 3 cards minimum gone to make your initial play.
If the card you summon gets negated in any way you are FUCKED. Period.

All the megalith cards (besides Phul I think?) don't have OPT's on their search effects and you can keep summoning them over and over to go through your deck adding but that initial play is so fucking weak and vulnerable to interrupts that they gave them the Ursarctic treatment and gave them a card which adds a ton in the new continuous spell just like Ursarctic Radiation to try offset that weakness.

I will say this though. The new misc Megalith is crazy and may be the one card that helps Megalith the most and I'm ngl as much as people hate misc this was IMO the only thing they could have done to salvage the archetype in any fucking way whatsoever because otherwise you start your turn, go minus 3 to summon 1 ritual monster then have it get Ashed and you just insta-lose unless the last 2 cards in your hand are broken Ithey won't be).

t. megalith enjoyer. Played Megalith/Fossils in one of our tournies forever ago.