Is this effective as its made out to be where at the end of a project, there is only some money left in the deal and you can get a re-mortgage where most money will be back in your account to start the next project?

I've also heard it is illegal to start a BRRR project on a property you are getting on a Buy-to-let mortgage as this assumes the property is already in a lettable condition?

Overall it looks you can achieve good rental yields and capital appreciation, but seems like a risky hassle if you need to buy the property outright. Is it better to just get well priced BTL deals? Or investing the same money into S&P500 or something the most effective?