1) We don’t have reliable life support machines.
Redundancy and easy part replacement should mitigate this. Parts can be produced on site so we don't have to ship parts from earth. It's chemistry mostly. We know what to do. It's just about good implementation.
2) We don’t know how to land heavy spacecraft on Mars.
10 to 50 ton payloads are not required, just nice to have. We could have a successful program with no more payload than we can already do. More payload isn't about capability; it's about lowering costs.
3) Radiation exposure.
Not the problem it's presented to be. Yes it must be mitigated and can be.
If this is the best arguments against they're pretty feeble.