My own experience and opinion.
But the manufacturer gives a range of accuracy, for the result.
But if you preform the test in the exact same procedure every time for example: the ph tester was already explained how squeeze pressure , will make a difference on readings, but from test to test what is the change, if you get 8.6ph two days in a row with the same procedure, id say its a good chance ph didn't change, not that your ph is exactly 8.6..other testers have more procedures wich leaves more room for variation in testing procedure. This could help make the tester look like garbage but they are hobby grade right, they just get us close and possibly able to notice trends.
Alkalinity tester example: i use the same covett every every time and fill it to the exact level, I use the same syringe for reagent with a visual mark. If my test to test numbers are the same the actual number doesn't matter until its way off from where I think it should be, then if it is i use another means of testing to verify what i think is going on. I know it could easily be a point or two off but the range is important, just because its a x.xx value doesn't mean its exactly on point, thats the reason for the manufacturer putting a +/- value on it.
This is my opinion, but I have no issues with their testers and don't believe them to be garbage, but I do believe they can be a good tool if used correctly they will do just fine to keep your water in range.