Mandalorians in my eyes are above average Infantry units in the Sith or Republic, and still above or on par with Elite Units or Weak Jedi/Sith.
People dislike the Mandalorians of the Mandalorian Wars, because they could kill a Jedi, successfully. Oh my GOD! A Jedi gets killed by a non-force user... Well, let's take a look at Order 66 when a bunch of clones almost successfully wipe the Jedi off the face of the galaxy...
Mandalorians are trained from birth, and the ones that make it to mid-age are Veterans. They've been through many combat situations, and in this period, they fought both with and against the Sith and Jedi. Several people throughout Star Wars lore have figured out how to kill Jedi. Person of Note, Atton. When you fight an enemy for a long period of time, study everything about their culture and fighting techniques, as the Mandalorians do for the Jedi, you learn how to best them. Regardless, does this mean a Jedi is on par with every Mandalorian, no way. But there are Mandalorians out there who can kill Jedi, as well as Jedi getting sloppy in battle and being killed. Hell, Jango was noted to have "slain multiple Jedi with his BARE HANDS" At Galidraan, and it was approved canon.
As far as Beskar'gam goes, the Lightsaber resistant materials required to make it, are rare. Not every rank and file soldier of the Mandalorian Armor will be wearing this. Or they would have won both the Mandalorian Wars and anything following in the Great War. Mandalorian Veterans and Elites are the ones who typically have pure Beskar, and some only have few pieces of it.
Mandalorians to me represent a culture similar to that of Sparta. Regardless of what Traviss says, I do not agree with the drawing on the Vikings. Mandalorians are raised to fight from a young age, just as the Spartiatai were taken at the age of around 10-11 for formal military training. The parallels go on to say that Spartans, except for a few instances (Which they were highly dishonored for doing so.) surrendered. Mandalorians are typically the same way, drawing on statements during the Mandalorian Wars. Malachor V was a rare and dishonorable event that occured, just as the Spartan surrender on Plyos. Which leads to another point, that they can both be defeated and lose. They are not invincible, and it's pretty much routine the Mandalorians, although "winning in the beginning, are majestically and heroically triumphed by the forces of good!"
Overall, in the lore of Star Wars, Mandalorians are used a character or army's legitimacy. Revan, made legitmate by besting them in the Mandalorian Wars. The Jedi in the time of Dooku are seen killing them at Galidraan, one of the SWTOR famous Bounty Hunters, made her fame by Killing Mandalorians, and the smugglers that defeated the Mandalorian blockade, were legitmized by killing Mandalorians. And in my opinion, they deserve much more than this. Why people hate them so much, is a guess but to say people hate the "Fandalorian" types who kiss Travis' feet and say Mando's can walk over everything that moves in the Galaxy. Just like people thought after watching the movie "300" for the first time... But in the end, as I look back at the post I just made, I consider myself a heavy fan of the Mandalorians. But just because I respect cultures like theirs from history and what they fight for in canon.
** Edit **
Also, the New "Mandalorians", are not Mandalorians. Just as Sith is not Jedi.