The bad reputation, in my opinion, usually comes from being highly drama-prone, as many people have said. I’ve personally seen military guilds that constantly fight with other guilds due to lore disputes/disputes over representing the same entity, such as the Stormwind Guard or the Sentinels. That’s usually fixed by establishing the guild as a separate subfaction, such as the Silverwing Sentinels or some specific platoon of the military, etc.
My biggest beef with certain military guilds comes down to their rigidity and authoritarian structure. Sure, it’s good to pay respect to officers and veterans, but forcing new guildies to gruelingly obey whatever a given officer tells them is not my cup of tea at all. Especially when many such officers tend to be far more interested in showing off how Cool and In-Charge their character is rather than crafting an interesting story that everyone can enjoy.
If you’d like to try a military guild, browse around and find one that looks good to you, join on an alt, and see if you like it. Military guilds tend to focus events around training drills, marches through the capital city, maybe some more casual “off-duty” events, and military combat events like missions, battles, a guild storyline, etc. but every guild varies.
Green flags for good guilds include friendly and available officers, a good number of events on the calendar (1-2 a week at least) run by more than one person, friendly/welcoming guildmates, an active guild chat/Discord, and clear, well-established rules that every member is expected to follow.
Red flags include distant/unfriendly officers, very few events on the calendar or events that are announced very suddenly, mocking or unfriendly guildmates, dead guild chat, IC events where people don’t talk outside their cliques, or a lack of established rules/rules that are enforced unfairly.
Another HUGE red flag is immature officers, or officers/guildies who seem to spend way too much time flirting or ERPing with each other. I have no qualms about people who want to ERP, they can do whatever makes them happy, but I’ve seen far too many guilds devolve into little more than harems with a uniform.