You realize that lore wise a space marine dies in one head shot from a bolter? What makes you think you can take 15 or whatever the hell you want?
I think I die really fast, but the main reason is that there are no weak guardsmen or cultists to slaughter, if you want to feel like a space marine go pve there are plenty of gants to slaughter, I usually get 250-300 that fits to the lore
Main thing is, that many LSM's believe that they are Chuck Norris or Rambo are and that they can kill everyone, but are shocked that LSM lose most of the matches. Being a supersoldier in this game, means also you need to think and work in groups to accomplish the objective's, but what the Orks should do, mindlessly attacking without consideration of the loses, its the LSM's that are doing it. What use is it if you have +20 kills, no regards about thinking to attack or defend the objective's and die continuely? There is better teamwork between the pugs from other faction's then bewteen the LSM.
Gameplay outweighs lore accuracy. This is a multiplayer game, so Space Marines can't wear all their GW brand plot armor just so they can shit all over the other factions. Besides, Space Marines are fine. They have better survivability through their gear than Eldar and Chaos...not sure about Orks, but probably them too. Their problem is half the time nobody wants to play as a team or coordinate effectively LIKE YOU NEED TOO IF YOU WANT TO WIN. Seriously, please don't just ignore the chat box. And stop running around with so many 1-3 man squads. Conserve your transports. Tanks are not taxis.
...it's a lot better than it used to be. Unfortunately LSM is considered to be a default faction that new players tend to lean towards (humans relate to humans, right?) and the loses that they inquire are chalked up to the lack of experience with the game. No matter what you say or how you say it they will always make the LSM the underdogs. If another faction is stronger then they are "more skilled" but if LSM become equal or stronger than another faction then they are OP.
I could feel like a space marines but and it is true for me with any other faction timer matter. If matches were longer or even without timer (exept fortress because without timer it is a win for attack) and cap points gave strategic assets (terrain scan, support fire like bombardment, spawn time buff,...) then I also could act more like a SM. That some map get timer for specific mission like defuse bomb, sabotage com before message is send us fine but to implement tactical gaming we also need time to regroup, deploy as planned (or as closed as planned because you never know), have attrition fighting... But right now timer make tactical second to rush and can't help to give tactical value to some of the basis of team play: wait for your team mate.