Adriaan.S

as pointed out by @Jaco de Jongh more details about the issues might better help members to provide opinions and maybe even solutions. 
I think many vague generalisations confound the matter a bit - I see the following 
At the very least there are many different makes of lithium batteries and from different makers there are different models. Some lithium batteries will work easier with some inverters/chargers than others. Some lithium batteries can seem a bit "fiffely" to set up. If a lithium battery BMS repeatedly disconnects the battery because some setting on the inverter is incorrect or the comms cable is plugged in the wrong port this can seem like a "battery issue" while it is actually an installer/user issue.
So, what is meant with "issues"? (Never been able to switch on the battery... battery keeps on switching off at some time during the day/night.... battery when fully charged only lasts 10 minutes but 3 months ago it lasted 8 hours and 6 months ago it lasted 12hours.. etc)? 
This is probably most relevant for you - does Northern Cape/Namibia type WARM temperatures break lithium batteries? In terms of total lifespan of the batteries it probably can but those same temperatures will likely do the same to lead-acids. 
All battery (including lithium) specifications are generally rated at a temperature of 25 degrees C and when going outside of this temperature the performance will differ. For lead acids there is a basic rule of thumb that for every 10 degree increase in average temperature the expected lifetime for the battery is reduced by 50%. (if the battery was designed with an expected life of 10 years and it is operated at an average tempertaure of 35 degrees you will likely only see 5 years from the batteries if you are lucky)
From a VRLA manual

Lithiums like any battery are not immune to heat. In one experimental study lithium cells stored fully charged (not best practice) at 35 degrees showed minimal capacity loss even after almost 2 years (still mainatianed about 90% of their starting capacity) but cells stored (again fully charged) at 60 degrees barely made it to 4 months (maintained only about 10% capacity).
The day-to-day use of the batteries is also influenced by temperature.  Temperature compensation during charging being especially relevant for lead-acids. If this is not done the battery lifetime will be reduced.  Many lithium batteries will indicate different temperature specs for charging vs discharging - charging having a lower max temperature (generally around 45 degrees C) than discharging (50-60). The lithium battery BMS might temporarily disconnect the battery during charging if the temperature is too high - while inconvenient at times it is not a sign of a malfunction - it prolongs the battery life.
With that in mind it is probably safe to say average temperatures above 40 degrees C will cause premature loss of capacity in any battery. (i.e. don't put any batteries in an unventilated corrugated shed in the full sun during the summer in vioolsdrif).
So, If we assume "issue" means the battery will not switch on, how do they know the heat caused the issues and not the 80km trip over a corrugated dirt road that shook lose some solder joints on the batteries (and the bakkie's bulbar...). If the battery capacity is prematurely low, is this because of heat or were the charging parameters perhaps incorrectly setup (maybe same parameters used as for lead acid?). Were these some once-of  no name imports or batteries from a fairly reputable manufacturer ?
They definitely are but at the same time some installers/users that are used to one thing sometimes struggle to use/accept newer/different technology. (someone referring to lithium batteries as Star Trek batteries sometimes being a clue).
While not impossible I would be curious how many of these 15 year sites there are compared to the number of 3-5 year sites. 
 
Lead acids could very well be the more appropriate solution  for you but that will likely be determined by more than just whether it is warm where you ive.