It is all trash talk my guess its mostly pvpers or people from other private servers trying to make homecoming look bad or something its very weird because objectively when you play sentinals right they offer allot more in teams then lets say a scrapper especially depending on the sets you choose, as they should be compared to scrappers not blasters as I have learned from comments and my own experience playing my sent.
Here are some facts, sentinals are amazing soloers, you can relax and even party to play this char, this is in no way saying its easy mode but you do not need to chase down targets much so in many ways it makes it allot easier.
A good starting at with cheaper ways to build them and more flexible then other ats for choosing powers and slotting. For example with a blaster just to cap defenses you must follow a very linear path constantly, this is because its an overtune no matter how we choose to see it. So when people claim blasters are better they are not they are a one trick pony unlike sentinel.
With a sentinel you are usually the last to die in teams, you provide secondary effects from your at to help your teams, and you can focus on other things to help the team like side enemies on your own and etc.
They are a seriously misunderstood class many do not even realize you should put that debuff clicky on auto it makes a world of difference and they get melee bonuses and they are meant to be the real blappers.