The HP bonus on Earth creatures has always confused me. It only changes the value of the creature on 1 creature (Hematite Golem). As such I try to stick to the middle of the +0 value range just in case the Earth bonus is a fiction created by Hematite Golem being slightly OP (I don't think it is but it might be.)
With all burrowing creatures I base the balance on both versions.
4|4 for 4
4 (4attack) +0 (1<HP<7)
or
2|4 Ambush, Burrowed for 5
2 (2attack) +0 (1<HP<7) +0 (Burrowed) +X (Ambush[5
])
Here I assumed X was equal to 3.
Now X is based of the average number of Ambushers that would be on the field averaged over the course of an average game. Ambush creates an average additional attack per creature equal to X(N)=(N-1)/2. X(7)=3.
Now I guessed that due to fractal, the average number of Land Wyrms on the field (due to the 5|4 actual cost) would be around 7. I would need a veteran player to tell me if this is accurate.