The armor being used on Mech's, even the earliest mech's is lighter and stronger than the armor in existence now so it stands to reason that a stronger cannon would be needed to do the equivalent amount of current day tank armor. It also, necessarily, explains why a 100 ton Atlas can stand 60' tall. A 100 ton mech today (assuming it was intended to be effective both offensively and defensively and not simply trying to be capable of motion) would likely not be capable of even half that height (The Abrams tank is the same weight as a Dragon and is only 24 ft long, which is about as tall as a Jenner). The reasoning behind the armor being equally viable against all forms of attack is that it is the lowest common denominator (See hardened armor, reactive armor, reflective armor etc... each being more effective against certain types of damage).
That said I think the biggest believability hurdle that Battletech is challenged with is the fact that understanding of the rate at which technology would improve over time was just not very well known in the 80's (nor, really is it now). We have already invented or have early proofs of concepts for many technologies which were not commercially available when Battletech was written and/or are not available until he far future in Canon.
- Myomer
Canon - Invented 2350
RL - Available soon (see EAP Electro-Active Polymers) - Guass Rifles
Canon - Invented 2590
RL- Prototypes exist now (you can even make one) and they will be equipped on Naval vessels within the next 20 years - Targeting Computer
Canon - Invented 2860
RL - The technology described in TW has been available since early 80's on the M1 Abrams tank - Rocket Launchers
Canon - Invented in 29th and 31st centuries
RL - Seriously? The chinese had them in 2000BC and militarized Rocket Launchers were used in practically every war in the last century - Jump Jets
Canon - Invented 2471
RL - What did they land on the moon with, springs? - And many more. Feel free to add on. It's kind of fun to look up Technologies on Battlewiki then google them...
Still fun to think about though