Actually it is far easier to tumble at any level, as long as you have ranks in Tumble. They can grow faster than BAB itself. IF they have ranks.
Two: if you hit with a Critical it is far likely you will knock someone dead. Especially if you have a melee weapon. (Don't) try this at home: pick up a bat and hit someone's head. You'll see that even a guy with little muscles (like myself) can knock down another person
. Critical hits, however, are an exception and more difficult to achieve.
Three: a BAB requirement not only makes no sense, it also would mean the feat would be selected at level 3rd instead of 1st so no true balance there. Higher BAB requirement than +1 or +2 makes no sense at all.
Four: melee combat follows the same tactic as mounted combat in D&D: high-risk, high-reward. You only get one attack but you can have that attack deal huge amounts of damage even for other characters of the same level. If you fail the attack, or the enemy manages to dodge it, you just lost an entire round. Also, the high-risk part comes from the fact that you become the greatest target of all which against multiple enemies will kill you.
If you do hit, however, you deal huge loads of damage at lower levels. But given the lack of additional means to continuously increase the weapon damage or gain additional multipliers, as you have in D&D, you will soon hit the maximum damage you can deal, which eventually is completely put behind by the large amount of shields that enemies have.
Various enemies also carry weapons that are dangerous at close range (shotguns, Heavy Weapons). Against those the risk is higher once more.
Five: in many situations you have to cross a large space to reach your enemy so melees have that disadvantage. During the time you are crossing it, you are also being targetted (most likely)
Six: the high dependance on Str means you will have other low stats. In this game this means less powers, less defense, less skill points, etc. Is that good? In some situations yes, in others (especially ones where you have difficulties reaching your target or there are far too many enemies for you to simply risk going in) not so much. In social encounters having few skills can be problematic. Now you may say that you can have good rolls when determining stats but remember that a game is made for the average, and the average is assuming the character starts with 74 points.
Seven: don't forget that there are enemies that can make a one-kill attack in melee. Like the YMIR or Banshee. I truly don't see melee attackers charging a Large enemy at all, too much risk. Against a Thresher Maw or flying enemies all you can do is eat popcorn while others deal with it. Against Klixen you are in trouble if you kill them (explode on deah and all that)
Eight: By level 10th you have a DPR (damage-per-round) far lower than ranged-focused characters. You also have fewer powers so other character have advantage over you on that. You will, however, have special attacks that can do cool and very useful stuff (the special attack that allows a character to change places with anoher is awesome)
Nine: If you remember correctly, in the ME2 and 3 games whenever you got close to an enemy in melee that enemy was dead unless he too had some powerful melee attack (which, if I'm not mistaken, occurred only in ME3). That target would die without doing much against you, and every melee attack done against him would deal a lot of damage even not using weapon mods that increased the melee damage.
Ten: some power mods (example: Flamer and Snap Freeze) make melee combat even more dangerous. While standard NPCs don't have them, GM made NPCs can have them (and will have them if there is a player that focuses too much on melee).
Various points against and many in favor of melee combat. Make what you will of it but the fact remains at high levels melee combat becomes far too risky and though still rewarding, it may mean death for the character. So at low levels the melee combat has to be better. Similar to the monk class in D&D 3.3: it sucked completely until it reached levels 8th and above. Here's the opposite: quite good until it reaches levels 6th-8th and then it's usefulness compared to others will not be so different and the risk associated with it will increase exponentially.
Retraining is not even an option since you cannot retrain ability scores. Spend many points in Str and the best you can retrain to is possibly a sniper. Thing is, those that go into melee spend themselves on Str and likely Constitution (if you are likely to lose your shields fast, you want to be sure your HP are high and don't depend solely on the dice rolled at every level). You will lack Dexterity for ranged attacks and other scores for powers. You also lose quite a lot of resources retraining, which is always problematic.
"This is all Joker's fault! What a tool he was! Now I spend all day computing pi because he plugged in the Overlord."