Charlie Bishop of Chadwick Model Railway has built a large layout involving a two helix design, one at each end of the layout.
His lower level is non-scenic and comprises a large fiddle yard. The helix design and construction feature in a number of his YouTube videos sometimes as main features of a video or sometimes as an aside during an update on some other topic, having watched most if not all of his videos a couple of take-away points that I have got from them are:
(i) The helices themselves take up quite a bit of room (as already mentioned by other posters on this forum).
(ii) Necessary to consider access arrangements to rear of helix (adjacent to walls etc) for cleaning, fault finding or dealing with simple derailment (the front layers of the helix get in the way of reaching in towards parts near the wall).
(iii) Consideration needs to be given to how to access underside of upper level baseboards, whilst it may be possible to lean/crawl in during early stages of construction, the presence of scenery, buildings etc on the lower level may make this impracticable once the layout is completed.
Here is a link to one of his early helix videos