A lot of the answers here are unsatisfying due to the implications that Orks and Tyranids don't also possess artillery, tank, and titan equivalents. It reads like a bunch of fan wank.
Ultimately the Guard has the ability to win by their tactical prowess, but win as much as they lose, often being on the defensive which is easier then being on the attack due to fortifications, and existential fear of death which can push these humans to fight fanatically beyond their usual abilities which don't exist for the Orks and Nids whom are monsters and always were fighting at their best.
Guardsmen put on the wall of a hive city are nearly impossible to remove, they are one of the best defensive fighting forces in the galaxy of armies built around the concept of assault. Not to say that the guard don't have their own ability to assault depending on the forces they have in any singular battle. Even without supporting from other human forces like the church, tech support, super mecha, and god angels, the guard can cover all their bases when they have the right leaders and supporting logistics.
If the guard have the right generals and proper access to supplies they are the ones of the strongest fighting forces in the galaxy, but cannot count on individual heroics like other factions to carry them if they don't have that. One guardsmen can rarely carry a war, but one super soldier can (as far as 40k narratives go.)
But it's not a sure fire thing in any event, not just for them but for their enemies. They aren't guaranteed to win even if they do have everything they need simply due to being outmatched on any particular battlefield or random chance that screws them over. Some guardsmen will arrive to a warzone not knowing how to speak the language of their commanders, or with wargear meant for totally different environments, to fight foes they have never even heard of before.
Combined arms warfare at best is how they can win. Knowing when and where to deploy the right countermeasures to the threats they face be they conventional or unconventional. If they charge with beasts, put them down with anti-tank lasers and missiles, they crawl up through the drains, organise hunter killer squads armed with plasma guns and melta guns. They bombard your fortress with artillery, dispatch counter attacks with cavalry, tanks, and helicopters to destroy spotters and unprotected guns where you can reach them.