The film's official novelisation gives us a little bit more information about the danger posed by the Droideka/Destroyer droids;
Although we only see a couple in the film, there were supposedly ten or more in the vicinity. Facing even one would be a challenge for a Jedi, let alone ten of these beasts. Ahsoka manages to destroy one in The Clone Wars : Landing at Point Rain by physically penetrating its shield, then turning on her lightsaber but this is clearly an insanely dangerous tactic.
The destroyers set up a "deadly crossfire". Since they're immune to their own blasts it seems there would be little chance of even a Jedi surviving a prolonged assault from multiple angles.
“Obi-Wan!” he shouted to his companion, who wheeled toward him at
once. “Destroyer droids!”
The younger Jedi nodded, smiling. “Offhand, I’d say this mission is
past the negotiation stage.”
In the hallway just beyond the area in which the Jedi fought, ten destroyer droids rolled into view. They resembled gleaming metal
wheels as they rounded a corner, smooth and silent in their approach.
One by one they began to unfold, releasing tripods of spidery legs and
stunted arms into which laser guns had been built. Crooked spines
unlimbered, and the droids rose to a standing position, armored heads
cocked forward. They were wicked-looking and deadly, and they were
built for one purpose only.
Skittering around the final corner to the bridge entry, they triggered their laser guns, filling the open area with a deadly
crossfire. When the lasers went still, the destroyer droids
advanced, searching for their prey.
But the anteway was empty, and the Jedi Knights were gone.