Advantages
Fits more people
Because many modular lounges don't have arms, you can fit more friends and family on the sofa together. The arm "ends" of two regular sofas with arms can equate in combined width to a space for at least one person, even two. So armless modular lounges can provide extra space for up to two people. It is a very popular option for large families and for those with many visitors.
It saves space
Got a long narrow room? You can put a modular sofa deep into the right angles of your room, thus saving space.
They're flexible (if you choose the right one)
Walk around the back of a modular sofa. If the back is one continuous construction, it is an L-shaped lounge which will have to fit the dimensions and spacing of your room. If it does, it looks like it was designed to be there. A continuous construction modular sofa is a hefty investment as it is designed to be a permanent "fixture' in your living room.
If the sofa is comprised of many, separate pieces, you have the flexibility to configure your seating in multiple ways. You might choose to separate a couple of the seats to act as freestanding armless seats, or reposition some of the segments into an L-shape or U-shape.
If you're buying a chaise lounge (the one with the extended seat) online, pay careful attention to whether it's a right- or a left-facing chaise. The direction is based on you facing the sofa, not sitting on it. So a right-facing chaise will have the extended piece on your right as you look at it.
Perfect for your forever home
If you're planning your forever home, a continuous piece modular sofa is perfect as you won't need to move anytime soon and you can plan exactly where it will go.
They're comfortable
The extreme length of modular sofas means people can sprawl out, lying full-length to relax. The long chaise section is particularly comfortable and literally allows you to put your feet up.
They're casual
Formal lounges typically have two- and three-seater lounges and an armchair or two. A modular sofa instantly dials down the formality and makes it clear this is a family home for relaxing and (literally) lounging about in.
Disadvantages
They can seem uninviting to guests
Most people buy lounges for their family to sit on and watch TV. This is fine in principle, however are you expecting your guests to use the same lounge as your sprawled-out teenagers? If so, your guests will be all sitting forward, facing the television. They will have to sit on an angle to talk to your family. A long line modular lounge can look like a hotel lobby if you're not careful (see photo).
Most guests "know" not to sit in the long chaise section as this is typically the favourite spot of one of the family members. So they often have to sit at the extreme edge so as not to intrude.
The situation is even worse when the modular sofa is not against the wall but has its back to those entering the lounge. This means its back faces guests walking in which looks visually daunting and uninviting; they feel like they're intruding, especially when they must walk around to "enter" the sofa area, walking in front of your family who may be watching TV. There is no faster way to make them feel unwelcome and hesitant about taking their seat.
You should consider having at least one armchair, if not two, to accommodate shy visitors who do not like having to "cram in" with family members on the sofa.
They're bulky
Sectional lounges take up an enormous part of your lounge room. This might seem perfect if you have a large lounge room, but visually they are incredibly dominant, taking over the room. They can also be difficult to decorate, requiring many cushions to soften the look and their sheer length makes it awkward to provide a suitable coffee table.
If they're positioned against the wall, it makes it near-impossible to include floor lamps or table lamps to provide task lighting, or even ambient lighting, unless you have wall sconce lights.
A one-piece constructed modular sofa - especially with a right- or left-facing chaise lounge in on the wrong side - can also limit your access to a doorway or window. It is rare to move to a home with exactly the same living room layout and dimensions and as a result, your modular sofa from your old home may fail to suit your new home.
Hard to move
If they're constructed as a one-piece sofa, ie the pieces aren't separate, they can be extremely heavy and bulky to move. While your furniture supplier may have carried it in, you will need people to help you move it.
Your one-piece modular lounge might fit your current home perfectly, but what about your new home? Your new front door or hallway might not be the right size for it to get through. If it has to go up or down staircases, does the staircase go straight or does it have a 90-degree turn? If so you might struggle to get it inside. This will, however, be less of an issue if your lounge is modular with separate pieces per seating section.
They don't suit all decor styles
Take great care to select a neutral colour as their sheer heft takes over the room. While perfect for a warehouse apartment or a modern home, a chaise lounge typically doesn't suit a streamlined mid-century look, nor does it look right for a British colonial plantation style decor scheme.