How much support you can expect to get from a multiway bra, depends on two things mainly: How you wear it, and how large your bust is. Yes, I know this is not exactly what you want to know, but it is true, Nevertheless, let me put things into perspective for you.
The best way to wear a multiway bra, if you want maximum support from it, is to not remove the straps and wear it as a strapless bra. No matter what the lingerie companies tell you, you will feel a considerable difference between “straps on” and “straps off”. Certainly, this applies if you have a large bust – more so than if you are an A or B cup of moderate size.
Think of a multiway bra as if it was a dedicated strapless bra – because that is what it will have to be, when you take the straps off. Does it have wide and rather thick back straps, perhaps with side boning in them? Does it close on the back with at least three horizontal hooks? How about underwired cups? If it does have these things, think about whether you would really want a bra like this. If you would mostly use it in its strapless mode, then you don’t have much choice, but all these reinforcements do not always make for a very comfortable bra. How much sense does it then make, if you only occasionally wear it strapless?
Personally, I would buy a multiway bra for its versatility, so I could in fact use it in all or most of its modes. All that heavy-duty side boning and other kinds of exo-skeleton-like features would only be in the way most of the time. This is mostly an issue, if you have a large bust. You may not like me for saying it, but in some cases it might be better to either wear a real strapless bra that does what it is supposed to, and then save the multiway for all other types of use.