I used to have a lot of conversations with Zeltrax before a lack of internet access saw him vanishing from the worldwide web. Among the topics we got into was his belief that there is no such thing as "tiers" in video games. Most of his reasoning applied to Super Smash Bros. Brawl, but we also touched on Advance Wars a few times as well. Considering all the shit I've done with tiers in the meantime (we last spoke in early 2011) I obviously don't agree with him, but some of his arguments are relevant to tier discussion.
The first part of the argument is simply about how tier lists are built off of certain assumptions. For a long time Grit was considered a powerful CO in AWDS, partly for certain assumptions about game mechanics, but we must also consider the builds in use at the time. I personally don't recall much talk about Recons as a viable defense until Gipface actually came up with it, and most of the games I spectated on in AWBW centered around Tank builds anyway. These AWBW players may have applied the same logic to AWDS; their conclusion is reasonable, since if you mostly neglect Recons in AWDS Grit will dominate. The other simple fact was that most people assumed air and sea units didn't count as valid arguments since they are optional rather than required portions of the game. At the very least, folks would downplay the importance of these units.
The second part comes in when certain rules are applied. Kanbei, when I last checked, is considered by WWN to be a broken CO. However on the map "The Coast" seen on the Tag Tier page, he actually suffers quite a bit. Any other CO on that map can gain quite a numerical and funding advantage on the guy, and Kanbei can't deliver the usual curbstomp (he might still win, but winning=/=curbstomping). Samurai Spirit is all well and good when the odds are fairly even, but Kanbei was designed with inferior numbers in mind; this map actually balances him quite well. I have gotten comments from WWNers on my map choices saying, "Well that map doesn't count because [blah]." I'm not necessarily disputing this; the point is that when rules are tacked on, objectivity is lost. Whether or not the map really is unbalanced, you have a case where only one CO out of 27 seems to be disadvantaged. It is not seen to bear considering because of tacked-on rules, though.
The final portion of the argument ties together the other two. Rules and assumptions can both be turned on their heads by simply... well, creating a situation where they don't apply. The serious incorporation of airports in metagaming by Gipface basically invalidated the basic Grit tactic by default; even ignoring anything else he supposedly had going for him, Grit's wall tactic was always intended for ground units, and anti-aircraft countermeasures took funding away from maintaining it. Now that the inclusion of airports has become a rule, however, any tactics specifically relying on them can be overturned by simply not including them. Similarly, naval units are under both an assumption and a rule: they are assumed to be subject to rock-paper-scissors mechanics and thus subject to the "first navy wins," and so as a rule are not metagamed in. This in spite of the fact that several COs clearly benefit the navy in ways that tip the balance in either side's favor; which, frankly, isn't terribly different from any other aspect of the game. Thus, when confronted with an actual naval scenario, any results garnered from the match will be played down to conforming with the assumption, and therefore be viewed as justification of the rule. Any other conclusion is seen as a sign of n00bishness and a general refusal to accept things that are seen as being obvious to anyone with a functioning brain.
The argument was primarily motivated by an old Super Smash Bros. Melee tier list, in which whole stages were deemed unplayable because one character could exploit the position of a particular platform and thus guarantee a win. I have to give some credit to the AW community that Zeltrax didn't: we, at least, will agree to ban a character rather than a map. Zeltrax contended that for every situation that community came up with where a particular character was overpowered, he could find another situation where they were underpowered; he claimed the same about Advance Wars games, and hence the above argument.
The relevance to tier discussion is that anyone who tries to go against the above three points is immediately dismissed. Now again this is coming from my own experience since I haven't bothered checking any AW forums in many months, but when GipFace tried convincing the AWBW community that their assumptions about AWDS, like Grit being broken, were completely wrong, he was dismissed out of hand for going against long-held assumption. Similarly, anyone going against GipFace's rules is dismissed out of hand for "not understanding how competitive play works." The thing is, these rules and assumptions are created by map-making norms, and map-making norms are determined by those who make the maps. If maps favoring, say, Grimm were the norm, Grimm would be considered an upper-tier CO. Sensei is considered good because of his relationship with cities, but what if a map has more bases than cities? If we decide that something is a competitive norm, it is only because of our ideas of what a makes a good map.
Now unlike Zeltrax, I'm not going to stand here and say that tiers are invalid. What I will say is that the preference for particular map-making styles has left open unresolved questions that bear considering. Yes, six paragraphs and that's all I really wanted to get across.