All releases are QA-tested, it's a volunteer effort, and the people doing it use whatever hardware they have available to them.
With more people stepping up to help, more issues may be detected, bugs raised & fixes worked out, but if issues aren't reported as bugs on a bug tracker (and also ideally testing-tracker), they don't exist (and for sure can't be fixed).
To me I get the feeling you have an issue with the kernel (18.04 using the GA kernel uses 4.15), 18.10 used 4.18, 19.04 used 5.0, 19.10 used 5.3 & 20.04 uses 5.4 (using GA stack) all of which are HWE kernels for 18.04). I had some [vintage] 2004 laptops (IBM Thinkpad r50p, t42p) that have issues with the 5.4 kernel but were fine on earlier kernels. Kernels are not a Ubuntu-MATE issue, and the issues I'm talking about occur on any flavor I tested.
Either way, all Ubuntu-MATE systems are QA-tested, and if you'd like to step up and help, that would be greatly appreciated 