ToB suffers in the normal things that push melee's limits (though not as much as other noncasters) - group battles, encounters with battlefield control/difficult tactical terrain, flying encounters, fights against casters.

And encounters above ECL 5 - ToB really is disproportionally strong in the early game.

But if your point is that it's less broken than casters, don't try to do it by making the case that ToB classes aren't powerful (they are); make the case by showing that casters are - Lesser Planar Bind a Nightmare and some Formian Taskmasters and abuse their at-will 9th level spell effects at ECL 9; build a gish that singlehandedly shreds groups of higher-leveled monsters (BC, miss chances. teleports and winning the action economy are key); drop a scry-and-die wizard who doesn't even give its opponents an action in combat; point at a sorcerer or psion breaking the action economy in half; use pretty much any summoner with standard action casting (eg. Rapid Summoning Conjurer, Linked Power Psion, DMM:Rapid Cleric) to lock encounters down hard.

But if you want to actually sell the players on using ToB (rather than just pointing at a few dozen of the areas where balance disintegrated), my best luck has been in running a game with the ToB classes on the table. Run some higher-powered encounters (but preferably not higher-leveled) where you get to begin to flex your optimizing muscles - interesting encounters with spellcasting monsters/enemies, tricky terrains, monsters that don't just walk up and trade attacks. This can demonstrate that ToB classes can be both challenged fairly easily and be fun to play.