Let's understand why we hear that editors like these multiple book concepts. For established authors in the business, this approach is great because you can hype up the whole series in advance and have the readers ready to buy everything the moment it comes out. Consider:
When we hear new authors getting those multiple book contracts, the editors also have a great benefit here (although the author does as well). If that first book does amazingly well, then the next books in the series can come in at a discount price for the editors. No, this is not a way to "scam some more money from the authors." This is a business gamble the publisher is making. Remember an advance is nothing more that a gamble the book will sell. If the book doesn't sell enough to recover from the advance, the book is going to be considered a failure. For the author, this is a great gamble because if the first book does poorly, they have A) a chance to see if the second book does better; and B) they will be getting that advance even with lower sales.
But let's get to the craft side of why writing a series can often be a bad approach.
First of all, I see far too many authors who start thinking about that series from the beginning. This, on one level is good because it does add in the plotting elements and those second books have much of that initial ground work in place so the writer doesn't have to start from ground zero. Unfortunately, many authors spend the time in the first book, not developing the characters who are on stage at that moment, but setting the stage for the later books. The end result is the story is often flat and dull. If you try to market that book to an editor, you are unfortunately not sending your best book out first.
The second issue is that writers are often trapped by the parameters they established in the first book. When they start into book 2 or even book 5, things have changed. They might want to take a plot one way, but those parameters are preventing taking that book in the direction it should go. Remember the story dictates where things go. It shouldn't be a prior book making those decisions.
Finally, and this one really hurts the readers, if the stories in book 2 or 3 depend on that prior book, you are forcing readers to read your stories in order. But think of it this way. You are at a conference when book 3 is being released. What do you have at the book signing? Book 3. What do you have in the book store? Probably book 3. Will the readers buy the book because you hype it up? Probably, but they will not like you because they simply don't know what the heck is going on with the first book.
Again, going back to the Bronwyn Scott series. These books can be read in any order. The guys do meet up with each other, but their encounters don't rely on a prior book to make sense. This is key.
My recommendation is simple for new authors. Write your one book. Think of it as a stand alone novel. Make sure to give your undivided attention to the hero and heroine. But, if there is a need for a follow up, keep that on the back burner. Don't plan it. Don't push for it. Write that one book at a time.