Metal Freeze gives the lead-popping power that the default Ice Monkey generally lacks. An Ice Monkey used for early lead-popping power isn't as reliable as alternate options, but it does come with the Permafrost effect for further slowing down bloons. As for higher-tier upgrades, this upgrade is generally reserved for Icicle Impale to damage DDTs in the absence of a Monkey Intelligence Bureau or without Acidic Mixture Dip spam.
It is not very reliable at popping Lead Bloons, but it is never a bad option for countering Lead Bloons, as its Permafrost effects can come in handy for certain occasions, especially versus incoming Ceramics.
Metal Freeze is generally used with Icicle Impale sometimes when direct DDT-popping power is required.
Adding this with Cryo Cannon allows a 2-0-3 Cryo Cannon to deal Round 63 quite effectively by popping the Leads and slowing the Ceramics.
The icicle spikes from Icicles are not affected by this upgrade, but only to the ice blasts.
Because of the Version 25.0 reworks, Metal Freeze is required for Snowstorm ability to freeze Leads and DDTs, unless given Acidic Mixture Dip. Likewise occurs for Icicle Impale.
Most buffs to Metal Freeze are simply inherited from Permafrost. No changes that are significant enough to change viability of the Metal Freeze upgrade itself.
Crosspathing in BTD6 (Top: Path 2, Bottom: Path 3)