I've never liked the replacing a vehicle with a crater/difficult ground. It's too undefined and arbitrary for me. For one, what is difficult ground? Difficult terrain is something; difficult ground is not (according to the rulebook). Right off the bat there's an issue there for RAW. Obviously it's just shitty writing and they mean to say difficult terrain. At least it's reasonable to assume so.
Let's forget about difficult ground though and go with craters. My issues are many. The first being the issue of "if you have one". My problem with that is it's not fair. Free 4+ cover saves for those who happen to have craters with them. Not cool. I guess I'm a communist; everyone should have craters or no one. I often play mechanized guard, could you imagine if I had a crater packed with me for every chimera in my list. Lame.
My second problem is one of size, the GW craters are huge, and almost always far larger than the vehicle that blew up. There are better solutions out there like micro art studios vehicle wreckage which is approximately 3x5 inches just a little bit bigger than a rhino. But still, the issue is that there is no guide for size. Anyone could pack a bunch of gw craters in their bag and bring them to a tournament. Nothing you could really say, the rules say use them if you got'm and they got'm.
Another issue with putting down a crater which is larger than the vehicle that blew up is it may prevent the remnants of a squad (assuming it was a transport) from being assaulted. That or the crater may simply not fit because of other models or terrain in close proximity.
Also, consider this. Before the game you define what everything is. You decide along with your opponent what is impassable, difficult/dangerous, what will count as area terrain or ruins and of course what save will be conferred by each piece of terrain. (you may or may not decide to go with the chart on page 21). What if there were no craters beforehand and as such neither side has agree to how they’ll work, where the edge is, if it is going to count as area terrain and what save it will confer and so on. The book will offer you a rough guide, like page 21 will tell you craters probably confer a 4+ save. But what about where the terrain starts and ends, is just the hole section of the terrain going to count or is any part going to be difficult terrain?
I just want to state that I like the idea of a vehicle blowing up and leaving a big hole for infantry to hide in. I like the idea of a changing battlefield over the course of the game and cover where there was no cover before. My issue is there's no standard for size, it's rife for abuse. If gw had a bunch of pieces similar to the micro art studio wreck pile for each vehicle I'd be fine with it. Obviously they would fit because they're more or less the same size as the vehicle that just blew up. There's still the problem of being able to afford enough for every vehicle in your army, again leading to situations that aren't fair. Also, there is the issue of real line of sight to take into account. You’d pretty much have to define craters/wreckage piles as area terrain to circumvent real line of site issues.
In a friendly/casual game this is likely a non-issue assuming you discuss it beforehand. I'm focusing more on in a tournament setting. What would you do if you were facing a big mech force and your opponent had gw craters for every one of their vehicles? Great you blew up his chimera/rhino, the squad is still getting a 4+ cover save in subsequent turns. Boourns.
Yeah, well, that's just, like, your opinion, man.
Q:Is it possible to be both a debbie downer and a negative nancy at the same time? A: Yes