Update explicit-failures-markup.xml

[skip ci]

status/explicit-failures-markup.xml

@@ -3515,6 +3515,17 @@ for more information.
             <toolset name="clang-linux-6.0~c++17"/>
             <note author="Niall Douglas">libstdc++ 6 is the minimum version which works.</note>
         </mark-unusable>
+        <mark-unusable>
+            <toolset name="gcc-6*"/>
+            <note author="Niall Douglas">Tests are unstable on GCC 6.</note>
+        </mark-unusable>
+        <mark-unusable>
+            <toolset name="msvc-14.1"/>
+            <toolset name="msvc-14.1*"/>
+            <toolset name="msvc-14.2"/>
+            <toolset name="msvc-14.2*"/>
+            <note author="Niall Douglas">Tests are unstable on VS2017 and VS2019.</note>
+        </mark-unusable>
     </library>
 
     <!-- phoenix -->