oh i like this topic, as i like a good moan!!
i have different reasons for disliking different brands at different times.
early 90's i just liked most things, i owned muddy fox/proflex/kona/orange, then i got in to the trade, i switched to GT as that's what the shop sold, then to Giant when i moved shop, then back to Orange when i changed shop again.
during this time many changes happened in the bike trade and in the ownership of various brands, both bike brands and parts brands.
muddy fox, obvs the name got bought and sold several times, and ended up being utter rubbish, this put me off the brand completely but i still have a soft spot for the earlier good stuff.
obvs proflex disappeared, so no real issue there, although i didn't like the K2 rebrand, just looked cheesy somehow.
Kona, first and only bike i ever won a race on, loved the brand in the early 90's, didn't like them in the Hawaiian flowery sticker phase, cheesy, then Halfrauds, ugh, that annoyed me, but later i liked them again and now i am a Kona dealer.
GT, got in to financial issues, in the Uk it was dumped in to halfrauds as a last ditch to get money in the bank, after it went bust and came back it was rubbish, it still bothers me to this day, for me the brands reputation never really recovered, i know they are much improved bikes but somehow it never captured that "i want it" feeling.
Giant, i always felt they were a bit generic, but i ended up working at a Giant dealer, it was after the DH team was out, they were sponsoring the ONCE road team, they became cool, now, i see generic-ness again.
Orange, i loved the hardtails back in the day, when the early full suspension stuff dame out i did not like it, the Mr O etc, awful! but once i was in an Orange dealer in 2003 i really started liking the brand again, the 5, the patriot, the re-issue clockwork, then the ST4, all great bikes of the time (obvs now i am a Orange dealer)
of the other brands, Specialized, was not really fussed about then ever, sort of off my chart, when i got in the trade another local shop was a specialized dealer and lets say i wasn't a fan of the owner, as a result i did not like the brand, when i ended up working in NZ the shop was a Specialized dealer, i found the bikes ok but the tyres etc and a few other specialized parts seemed to be a bit crap, but on the way back from NZ i stopped in America and got to visit their HQ, i was really surprised, at Specialized HQ i found bike nerds, i did not expect that. going there changed my perception of the brand because of the people i met, that said, i get the corporate thing though, i remember the Cafe Roubaix thing, but Mike Sinyard did sort that issue out to the owners satisfaction, i know this as i asked the guy about it and he was really open about it.
trek, always found them dull, average quality, then Lance, i hated that whole situation, was gutted when i felt "i think he's a doper", but like everyone else didn't want to believe it, the damage he did to Greg Lemond was disgusting, not to mention everyone else he bullied, as has been said treks response was extremely poor. for me it has switched me off to that brand completely.
in modern times, i can't stand Canyon and YT, not just because they are direct to consumer, it's just i don't think they are very good yet a lot of people think they are good, to me it's like why can't you guys see this? i feel they are designed by accountants.
i used to get moody about cannondale, with all their specific parts and standards, but now i think they are no different than all the others that do the same!
parts wise, i have my issues with shimano but overall i like the brand, early 90's dx/xt/xtr was great, i liked m737, m739, m950 etc, 2x10 xt was good, i like 1x 11 slx/xt/xtr, not overly fussed about 12 speed though. and yes i think microspline sucks, especially their own hub design, have you ever stripped an SLX 12 speed hub?! what a nightmare! Di2, when it's wireless completely i'll consider it.
Sram, i am constantly disappointed with Sram, i find the people that love it happen to be riding XO and above, but the more basic stuff is rubbish, comparison to basic shimano and that still works which is why it annoys me that basic Sram fails, snapped levers, flexy shifters, not to mention the brakes, good god why haven't they changed to mineral fluid?! why make the basic brakes with a master cylinder assembly that they know fails? they make an upgrade part, but it doesn't make financial sense to fit it! and they won't fit that part as standard! in certain areas they are choosing to suck! i do however think their electric system is good, but i don't trust the parts it's put in to. i also hate the XD driver, the bearings in the freehub body are too small and they fail.
Campagnolo, used to love it, now i think it's pretty dull looking because it's all carbon, translation, i think it looks boring.
i agree with the comments about Lezyne, i have seen so much of that stuff simply just not work, a friend got his brand new pump out, it failed, i gave him my 15 year old topeak pump which worked. tools that rust really quickly, headlight brackets that don't hold the light in place over rough ground despite being off road lights.
i have mixed opinions on muc-off, i think some of it is good, i think some is well . . . er . . . crap. i think people get brainwashed by the brand adverts.
what i really hate though is "standards", the annoyance of "ok we need to standardize bottom brackets" and the answer is inevitably, so now we have another bottom bracket standard! it's maddening!!