Doesn't fix all type arguments
In a C# 7 compilation, class C<T, U> : A<T>.B<U> results in a base type A<T!>.B<U>.
A test that hits that is ErrorTypeSymbolTests.ConstructedErrorTypes.

Also, UsesIsNullableVisitor probably should have caught that.

Incorrectly replaces nullable types with oblivious
There is another issue in SetUnknownNullabilityForReferenceTypes. It replaces string? with string~, but is should leave it alone (see UnannotatedConstraint_Override).

Produces different type symbols
Finally, it changes the specific representation of error types (produces a SubstitutedNestedErrorTypeSymbol instead of leaving an ExtendedErrorTypeSymbol), in that same test (ErrorTypeSymbolTests.ConstructedErrorTypes).