They’re usually okay, but they’re the same as a lot where “in-stock” can refer to what the distributor warehouse stock level is telling them. They are promising shipping dates based on how quick they can get it from them and out the door. I’ve had it before a few times where various places were in-stock (Biketart may have been one of them, not sure) and tried each one, all turned out to actually not be in stock. Distributor didn’t have the stuff basically and their stock level was telling fibs.
The only ones that used to be accurate were Wiggle and CRC, but since the merger and integrating their warehouses and stock system, that’s all gone tits up.
Though Amazon is increasingly stocking bike stuff and their stock system is usually spot on, plus they offer Prime on some stuff. Actually next day, if not even same day delivery!