It merely adds to the so-called slippery slope mechanism present in every strategy game ever created.  Once you are behind, you are likely to fall further behind as enemy comes back even stronger.

It's already quite enough of a penalty to be off the map for 30 seconds.  If it was a big team fight, the whole team might even get killed which will lead to multiple towers falling rapidly to a coordinated and unopposed push, possibly with a critical flag cap (+30% reinforcement dmg/health) and lock.  The game could well be over after a single kill towards the end because of this..it seems silly to additionally award the killer and all assisting teammates with gold.  Exaggeration or not, this would also help to lessen the effect of questionable AI and human players that lead to one team gaining a 50 billion gold advantage in 10 minutes with mageslayerahskandorallfathersringmassroflcopterwithhellfiresandrocketpods.  Oh and in order to keep the economy balanced, the passive gold rate could just be increased.

The second thing is that minions are insanely powerful.  Early game minions are cheap, low mana, and provide a relatively huge dps boost such that an assassin can't do very much to hold a lane vs an army.  Since the general must use mana only to summon at this point, money is saved for better items (not the v-something helm) like say, slayers gloves, so when the assassin comes back with AoE and a mana helmet to spam mines or whatever and hex scroll he finds that he is outclassed in a simple 1v1 fight because you just skilled slows and nukes and bought damage/health.  Should he not get AoE, the general still has his mini-army, along with simple creep support that now is not subject to AoE.  No way can an assassin take the dps of several soldiers, archers, siege archers, and priests that will all be present (but no creeps on his side as the opposing general already wiped them out), I think that is what leads people to underestimate minions.  It's not that you are supposed to get level IVs with every gold piece you have for the first 15 minutes just to get frost nova'd and get singled out with your 2000 health, it's that you will remain ahead with the damage threat that keeps the enemy back until they can counter by which time you already have the counter-counter while STILL having the original threat otherwise.  QoT out of the gate with shield, 2 archers and 2 shamblers is unattackable and does great AoE vs creeps.  Focusing then on spikes and NOT shamblers will throw the opposition off because they still have to deal with minions/creeps with AoE of their own, except that means they have become less effective direct assassins, and are about to lose 1000 armor and get focus fired by everything in the vicinity.

The net of all of this is I feel it's too easy to gain the upper hand as a general without actually commiting yourself to any kind of counterable strategy.  Generals can be half-assassins and half-generals with the threat of full generals, but assassins that are half assassins are just bad.  Probably the best example is UB vs anyone.  Let's say I buy priests and skill into whichever DG-specific minion.  I've already won any sort of direct confrontation as you will be half dead by the time you reach melee.  With spit, you've just given my priests something to do.  You have zero chance of holding a flag until your ooze is high enough to kill minions and creeps faster than minions and creeps will kill you (which is actually never).  You are therefore relegated to the role of tank in a team battle.  Supported by oak shield, you will be unstoppable late game with your permaslow and high dps.  However, team battles provide many opportunites to use things like slow/stun/heal anyway, so really what good does UB do past what any other hero could do?  It's sort of like having a weak rogue in a WoW raid.  You bring dps, but so does everyone else and then some.  You also die too easily.

Thus, point 3 is UB seems weak.

There are additional points but this is long enough.