Yes landmark global does have bad reviews on trustpilot. Most couriers have bad reviews on trustpilot basically because people only provide feedback after a bad experience. People are unlikely to say how wonderful their delivery went if everything was ok.
At the time of writing (2021)
FedEx had a trustscore of 1.5
Landmark 1.4
UPS 1.6
Parcelforce 3.3
DHL 1.4 to 2.8 (depending on which on website you picked)
We have been running 2 e-commerce firms since around 2000. We have used most couriers. We have switch to landmark global because they are more reliable than the last courier (royal mail). Lost parcels reduce. Delivery times were shorter etc.
It is an economy service. Its cheaper, slower than some, the tracking updates less often and it can be difficult to contact customer service. But it is overall a very good service for the price.