Hello!
The Peg-Pérego Unico is one of the cheaper/ little bit more budget stroller systems from PP brand. It shows - e.g., the handlebar is made of foam. As you mentioned, the simple 2in1 seat is positioned slightly lower and doesn't offer that much space for the child - mainly in the seat unit configuration…The chassis is almost identical to Book 51 seriesm which is nice, but there is a small problem - yes, it may start rattling over time, and moms owning those are generally not very happy about this. However, as a first stroller, for example, for the first year of life, this could be an interesting option - mainly if you get a nice offer (price-wise).
Just for inspiration, here are some higher quality stroller systems of the same type I would rather consider: Maclaren Daytripper, Diono Quantum...
I would not recommend Mamakiddies systems. In my experience, it is a very cheap (and cheaply made) brand, and I don't like the fact that they are always giving you an impression of some temporary sale/offer, but in the end, it is not a special price - those strollers are simply cheap all the time. They are very cheap, and that's quite obvious from looking at the finish and design... pretty much ordered from Chinese wholesalers. They are not worth it, often almost useless, give you potentially a lot of headaches if problems arise, and later you will have to replace it anyway... I would rather recommend a second-hand stroller system or search for one on sale - much more than those cheap Mamakiddies models... Rather than Mamakiddies systems, I prefer the original idea of having the 2in1 model, so you can replace it after a year /or a year and a half/ by a lightweight, non-reversible stroller (you can find many bargains in that category).