Strollers are allowed, but as you've figured out, they have to be folded before you get on the bus. There are no limits to the number of strollers, but scooters are limited to two per bus due to having to be secured in specific spaces on the bus. So when someone rolls up at busy times like park opening and finds that there are two scooters ahead of them, they know they'll be waiting for the next bus. And if the bus is shared with another resort (the BW Resort bus is not shared anymore, at least not at park opening), it's possible one or both of the
scooter spaces will already be occupied, making the delay even longer.
BTW the mention of Swan/Dolphin guests sharing BW and YC/BC buses is due to the fact that Swolphin doesn't use Disney buses anymore, they have their own buses that drop off at TTC instead of at the MK bus station. That means those Swolphin guests then have to take the monorail or the ferry to get to the MK entrance, and many of them prefer to walk over to BW or to Yacht Club in order to be dropped off at the MK bus station and avoid the monorail/ferry trip. It has made the BW and YC/BC buses more crowded at busy times of day.