To bucket or not to bucket (seat): the pros & cons of an ergonomic, L-shaped, bucket seat stroller
TL;DR: An ergonomic, "hinged", so-called bucket-type seat unit shaped like the letter L (or V), even if reclined, scares many conservative parents - as it is not a bed-like, lie-flat position per se. Be aware it's not suitable for the smallest babies (of under 5-6 months of age), but other than that, it was approved by doctors as well as universal standards - otherwise, the luxury brands would not use that style, damaging their reputation by an unsafe product... The opinions, of course, differ, and everyone prefers different things. It's going to get down on only your feeling about it and your child's preference (heavily influenced by that feeling).
The Bugaboo brand came up with the bucket-seat design in 1999, and it's still used today... Do you think it would be present on today's ever-changing market if it were a worse way - or even a dangerous one? Why would a luxurious, super-expensive brand damage its reputation by clinging to a design that would be unsafe for the health of the baby? I am pretty sure they wouldn't, given that they can (and did) produce a lie-flat model (the Bugaboo Bee). From another point of view, the European safety norm EN 1888 also approves of this style, and given the states don't really have much to gain by approving a not-as-healthy-as-it-should way of creating strollers, it truly comes down to the "it's ok to use it."
There are a few important advantages to the V-shaped style stying so even in the lay-flat position.
The first would be - it is a natural position for a baby to have bended knees. Just imagine how it was in the mother's belly. All folded, feet bent.
>The baby feels safe and secure when not completely flat and "exposed."
Another good thing about the bucket position would be its belly-soothing properties. To have your legs a bit bent creates a relief position for the baby. That is probably why the position is called an "ergonomic" seat as well - fewer stomach cramps or other inconvenient belly discomforts.
To add to that, when the baby is not all straight on a fully flat surface, but many of them experience fewer reflux problems. The slightly-inclined but still naps-friendly position of the backrest helps the mother's milk (or formula) stay in the little one's stomach. The same goes for when the baby's got a cold, experiencing a blocked - stuffy nose (nasal congestion). The "snot" doesn't flow back but out; the bucket-seat position helps him or her getting the mucus out rather than in.
The last more or less important advantage I'm going to acknowledge here is that the ergonomic position allows for a more stable laying as well as sitting position, and even a better view. Being slightly inclined while supported by the bent legs and knees, the baby "sits" more securely in the seat, not sliding down if partially reclined (as often happens in the traditional lie-flat reclining seats). And even in the fully reclined mode, it sees out better thanks to the incline and the seat's usual openness.
As I learned with my first son, who hated being "entombed" in the carrycot, a bucket-type seat is a very nice solution for those babies unhappy in a bassinet and so much happier when being treated to a (bucket-style) seat unit.
The negatives of the bucket seat (of course, there are cons, everything has two sides), would also encompass a few important points.
The first minus I got to see is the moms being scared of these "modern" solutions so far from what they know = lie-flat "beds" of the retro strollers as well as them being used to sleep in a bed that is by no means shaped. Why should their baby sleep in something else? Well, it doesn't have to, of course, the seat style is your choice alone. I, however, noticed that babies of these moms that maybe even like the bucket-style-seat strollers and even get one get, eventually, subconsciously nervous and worried if the baby will be ok in it, transferring the stress on the baby in the end. The result being that the baby truly not likes the stroller and is quite nervous in it.
Another disadvantage of the bucket seat is that it doesn't allow your baby to sleep on the side or on the stomach. Not many do that in the seat units overall - it is more difficult to belt them in properly those positions, but if you have a baby that simply prefers that, you are going to have a problem once passing on to the pushchair mode after the carrycot.
And lastly, the style and design of a bucket-style seat unit makes it a bit more open, as well as a bit smaller in size (not always, though - the Baby Jogger City Premier is a bucket-style-seat pushchair with one of the largest reversible seats overall). The smaller space inside these reversible seats is caused by their reversibility just as much as their shape, AND also based on the canopy being right above the backrest - because it needs to be secured that way (but is often designed like that even in lie-flats seats). It is not true that bucket seat equals smaller seat, but it CAN mean that, mainly with the cheaper, simpler stroller models. It also usually doesn't offer a leg cover (apron), and the choice of the footmuff will be best made if the footmuff is L-shaped too (Bugaboo footmuffs, as well as my favorite Lodger Bunker, are made like that and fit the ergonomic seats much better than the flat-backside-design footmuffs)
So, should you choose a bucket-style seat unit or a more classical lie-flat that offers an individual backrest and leg rest adjustment up to a fully flat position? This is, as everything, highly individual.
If you simply feel bad about a shaped seat, don't buy such a stroller. Your baby will feel your subconscious dislike and might dislike it too. If you don't really care, or even like it, look at other stroller characteristics, too, then - you'll always do better with a higher-quality, known brand with good reviews (not necessarily more expensive, even if often it does work like that).
One more thing. "Don't put your newborn in a bucket" unless you buy a seat insert making the seat flatter - not because of the shape itself, but because the newborn will be too lost, too folded in that seat, being so small the legs won't get bent where they should, not being able to reach there yet. A firm, flat surface of a pram (bassinet) is best for newborns since they can curl up or even lie on their belly there while fully protected from the outside.