Equipment- and training-wise
they were a step ahead of most of their neighbours. Rectangular shields that easily interlocked combined with the comparatively short stabbing weapon, the gladius, invited the enemy to come in close and fight on the inside; while they were attempting to break through the wall of shields, they invariably found themselves being mortally stabbed by a weapon flashing out from behind the wall. A century, either on the move or defending itself, was a difficult unit to break down.
Tactics were good, strategy was not always so, marching a legion or two forwards, head-down with the intent of punching a big hole in the enemy's line usually worked, not sophisticated or elegant but it worked. The technique came unstuck big-time at Cannae against a clever, mentally agile and tactically mobile enemy.
Hannibal learned the hard way that when you've got the Roman Army on its knees, you don't let them back up again.
They moved with the times, so yes, definitely adaptable. They incorporated new weapons and learned from previous defeats, all were rolled into their order of battle so as to avoid making the same mistake twice. They deserve credit for seeing the importance of, and incorporating significantly into their campaigns, the forerunner of the combat engineer in fact all line soldiers had to possess building skills to include construction of military-application structures.
Given conditions of their choice, they could and did take on armies that significantly outnumbered them, and won. Boudicca considerably outnumbered Paulinus's force but in allowing him complete initiative was utterly defeated.
The Iceni learned the hard way not to let the Roman Army choose the venue for a punch-up, there was usually a reason for it.
They came to depend far too heavily on foreign soldiers, I doubt this was the killer blow to the Roman Empire but it very likely didn't help.