I'm sorry in advance for this novel-sized reply.
I’m a long-time bucket fan for all phases of fermentation.
I use buckets for everything from IPA to lagers to lambic. Primary, short conditioning (‘secondary’), and long-time mixed fermentation.
For sours, I’m often criticized for using buckets (and even by the Mad Fermentationist himself), but I think a few practices significantly limit oxygen pickup.
1. FILL IT UP. I try to get at least 5.25 gallons in my 6 gallon bucket. My batches that turned to vinegar have been due to under-filled fermenters (both buckets AND carboys). If I’m using a big krausen producer in primary, I use Fermcap and have never had issues with blow-off. For lager strains, most bretta, and 3724, there usually isn’t enough krausen to worry. (Side note: I think blow-off tubes are a potential source of contamination, but that’s another post)
2. Replace lids often. For long-conditioning beers, I won’t use a lid that isn’t a PITA to remove. It doesn’t take too many cycles for a lid to lose its shape, so those lids get marked “MALT” and are used for grain storage. I’m currently testing some lids with O-rings I found at the LHBS; I’m not yet convinced they fit more or less snug than the regular lids.
3. Limit opening/sampling. This is probably the reason most homebrewers have issues with buckets. More sample = more oxygen and more wear on the lid seal.
4. Keep airlocks full. An empty airlock will allow more oxygen in than the lid. I’ve switched to 3-piece airlocks for conditioning buckets. Also make sure that the grommet fits tightly around the airlock. Don’t pull the airlock to fill or ‘peek’.
5. Healthy, active cultures. I propagate dregs and mixed cultures so they are healthy before pitching into wort. This greatly reduces the overall aging time. My sours are usually ready to keg in under a year, but I’m confident that at least some bretta will stay in suspension for much longer. For clean beers, the same rules apply: healthy yeast and fermentation conditions allow the yeast to protect the beer from oxygen.
6. Limiting acetobacter exposure. You can’t make acetic acid without oxygen, alcohol, and acetic acid bacteria. I try to minimize acetobacter in my mixed cultures by stepping up dregs in the bottle and tasting before adding to a culture or batch. I also regularly feed the cultures, which help push oxygen out and keep bretta dominating. It also helps to keep the exterior of bucket and lid clean.
6. Flush with CO2. If you can, flush the bucket before transfer or even after opening. This is less critical, and I'm guilty of skipping it on occasion, but it will help (again - with buckets or carboys).
WHEW! Sorry again for the diatribe, but hopefully this helps some of my fellow bucket brewers!