There are certainly some properties that aren't up to snuff. Those tend to be the hotel conversions or older resorts that are not as consistent with the brand. The Mayflower is a hotel conversion. I am not sure why you would say the Myrtle Beach location was not up to standard. I suspect you are talking about OceanWatch? This is one of Marriott's flagship properties with beautiful 2BR villas. The location isn't on the boardwalk of Myrtle Beach, but many like the location in North Myrtle Beach. The Marriott OceanWatch resort is far and above better than Sheraton Broadway Plantation.
As for comparing the different resorts; Sheraton, Westin & Marriott. We own both Marriott and Vistana and have stayed at over 30 resorts. I would put the Westin resorts on par with most of the top tier Marriotts. The problem with many Westins is the amenities in the units, especially the kitchen is lacking. Poor quality cooking supplies like pots and pans along with a poor selection of cooking utensils is one thing we don't like about the Westin Vacation Club properties. Westin and Sheraton don't even provide any food storage containers in the kitchen. Marriott's have higher quality dinnerware, silverware and pots and pans than both Westin and Sheraton. As for the rest of the unit, Westin is far and above better in the bathroom department. Their villa bathrooms are fantastic. Marriott has an edge for in room entertainment given that most of their units have internet connected television systems that allow you to connect to Prime, YouTube and Netflix without needing to have your own device. I have yet to see that at a Westin or Sheraton property. Westin and Sheraton also use flat sheets on the beds and don't seem to have a proper duvet. We always have to pack our own fitted sheet when we know we will be staying in a Vistana property.
As for the rest of the resort; Marriott, Westin and Sheraton all seem to compare about the same. Onsite food and beverage is about the same but we tend to notice that Marriott usually has few more options and I give the Marketplaces at Marriott resorts a bit of an edge. There are of course outliers on either side where a resort isn't up to par or where a resort far exceeds. Westin Kierland Villas is like this. The resort and overall feel of the resort more than make up for the shortfalls that exist in the villas (kitchens without ovens and low quality supplies).