I have a flat 80 watt panel on the roof rack of my Defender which is working just fine at charging the 110ah AGM battery to run the Evakool 35 litre fridge.
I camped for a week at Christmas and never had a worry.
Having the panel on an angle is only an advantage if it is pointed at the sun all day, which doesn't happen. For part of the day the panel will not be pointed at the sun and so may be less efficient than if it was flat and still getting some radiation.
For example, how much of the day does a vehicle spend driving north to get full radiation from an angled panel? For much of the day it will be driving east, south or west and the angled panel will be useless.
This is why some houses have some panels pointing west for the late afternoon sun and others north for the midday sun.
2009 Defender 110 2.4. ARB bulbar, Ironman winch, Safari snorkel, Steinbauer chip, Lightforce spots, Long Ranger tank, Off Road Systems drawer, Traxide 160 controller, Tekonsha brakes, Mulgo seat runners, Uniden UHF, Nuggetstuff seat corners, breathers.