The only real issues impacting performance of (all) portable units is when it has to rely on dead reckoning - built up areas (proper built up areas, not little villages), wooded areas, underground roads etc, or if you have heat reflective glass which blocks GPS. Portable units don't have the sensors to cope with that, and even phones are using older generation of GPS chips (standalone portable units generally much older).
But if you don't have a factory fit option, its usually your only choice. But the positional accuracy of the 20yr old CARiN far exceeds that of the most modern portable unit.
Sadly, I do know far too much about GPS chipsets and protocols used, due to some other interests and hobbies I have. Generally, you need a minimum of 6-8 sats to get a proper lock. Most portable units will claim to have a lock around 3-5 sats, when they don't, and by their nature in a car, they will struggle to get into double figures (more sats, better the accuracy, and less reliance on weaker signals). A modern chipset should easily get 20+ sats in open space.