The current behavior of digging is that shooting projectiles on earth/red tiles will roll for a cave-in, whereas melee attacks don't do this. Presumably this is to emphasize the safety of 3-tile digs: you can dig that type of tunnel with a melee or ranged weapon and it's perfectly safe, for the ranged weapon you'll have to stand outside the tunnel and fire thrice (accurately).
There's a few issues that result from this, the main cheese is that you can dig a melee killhole and fight a large group of enemies 1-on-1 while they're suffering cave-ins on their own tile. Hunters sort of stop that ideal scenario, but it works relatively often. You can even do this with siege mode, which prevents bump-attacks but you can still target with 'f', that ends up feeling a bit tedious.
The main imbalance arises with imprinting: it's designed to have tons of legs synergy over say threads, but there's no strong reason to prefer legs over meleehover in Research and beyond when meleehover can deal with any amount of attrition by digging killholes, just gotta direct Hunters to Zionites or launcher them before killholing everything else. The 20 Swarmers or Grunts you fight in a row 1-on-1 can't really do anything to the integrity of your cmb. hover and melee weapons if you have even slightly reasonable utilities. In a way that is the supertankiest build you can play, in part thanks to killholing but of course also because the vulnerability of lategame cmb. hover and melee is rather low.
I would challenge the notion that the safety of melee tunnels is at all necessary, tunneling doesn't need to be that safe, where you can melee a diagonal to escape LOS before the tunnel is even complete. Flight and hover are fast enough, they would be more exciting to play if you could only melee through 1-tile walls in safety, fast+infowar is currently super safe and any nerf to its simple play patterns would be nice. While some tedium could arise from this type of build wanting to switch to a ranged weapon to build its tunnel, these builds actually enjoy it a lot to keep a melee weapon in their slot and not really do weapon-switching, there's a real cost for them to both carry a ranged cannon and switch to it and fire 3 or more times, the time and resource costs both represent real pressure.
This is also some kind of nerf to that thing threads builds do where it's genuinely less attrition to de-equip fragile parts and tunnel 30+ tiles to e.g. a hacked exit rather than dealing with patrols. You would still do it at times by first opening a 16+ tile tunnel with a ranged weapon, but there's no need to completely squash that cheese given it's at least mildly appropriate for the slow & tanky build.
