Yes it's a bit extreme/scaremongering!
When you look at ADAC crash test results (the supplier of the EU consumer magazines e.g. Which) you do see that in general the all stages or multi stage seats (covering rear facing, forward facing and booster stage) represent a compromise in safety compared with seats that cover 1-2 stages. That's true even from brands like Joie and Nuna, who otherwise have high performing seats.
The scoring goes from 0.5 - 5.5 (if you like you can "translate" it to school grades, A+ all the way down to E/F) - All Stages seats in general typically get safety scores in the region of 3.5 - 4.5 which is about a D.
Spin type seats from 0-4 years (ie, without the booster seat mode), omitting purely rear facing versions, tend to score more in the region of 2-3 for safety (roughly equivalent to a B- to C+ grade)
High backed booster seats from 4-12 years, omitting those with a backless option, typically score in the region of 2-2.5 so approx a B to B-.
The Silver Cross all stage was unusual in that it scored 2.5 for safety (B/C), which puts it more in the region of an average performing spin seat + lower end (but OK) booster than an all stages seat. No other all stages seat has ever done this well. If you absolutely must have a spinning all-stages seat, the Silver Cross Motion All Size is the best one to have from a safety perspective, but it is very expensive.
Caregivers are often attracted to the models which are sold online or sometimes in cheaper baby stores like Smyths and Halfords, which boast a list of features as long as your arm:
Spin
From birth
"Extended" rear facing (beware, sometimes used even for 13kg RF limit)
Side impact protection (again, not a protected term)
Isofix
Seatbelt fit
All stages/up to age 12
Having an "i-" somewhere in the name making you assume it's i-Size
Recline
Universal (just means it can be seatbelt fitted)
Appealing name/cover design
£100 - £150 price tag
Unfortunately, it's just not possible to do all of those things well at that price. You're definitely getting a compromise somewhere. These seats tend to be mass produced in countries where the labour is cheap and imported over.
While I think it's a huge exaggeration to say these seats are "incredibly unsafe", they are often flimsy, poor quality compared to premium brands, confusing/hard to follow/badly translated/minimal instructions (a problem because incorrect installation or use is one of the main factors implicated in child car accident deaths) They typically get lower scores in the independent crash testing than seats from brands such as Joie, Nuna, Britax. (Silver Cross use a generic manufacturer for their seats, but seemingly a higher quality one, most of their seats have done well in their class). They often do not fit children well at the edges of an age/weight category, which means newborns often have too much space around them and parents are likely to move children to the next stage (e.g. rear facing to forward facing, harness to seatbelt) at a younger age because for example the harness does not fit well, and they very rarely truly accommodate children up to the height of an average 12 year old because the booster mode doesn't usually extend up very high at all. Sometimes there are ease-of-use annoyances like the seat being heavy, taking up a lot of space, or the fitting not allowing the spin to be used.
But - assuming you haven't accidentally bought a fake - they do conform to the legal standard, and if used properly they should protect children from the majority of accidents. Children are better protected in a higher rated seat, but these cheaper seats (provided they are used properly, and conform to the required standard) are not unsafe.
For a grandparent seeking 2x 360 seats from birth I'd recommend the Graco Turn2me i-size or Joie Spin 360. It doesn't come in pink unfortunately, but it does have a very high safety rating, isofix, spin function and a low price tag. They can be used up to approx 4 years old. If the older child is already using a booster seat, the Joie Trillo LX is a good option on a budget and likely comes in a matching colour. Or Britax Evolva is a good option for a child who is over 2.5 years, forward facing and will need to come out of the 5 point harness in the next year or so. There used to be a pink version of this, possibly some retailers still have old stock.