Just not legal for RULEBOOK combat events... There are clubs running non-rulebook events (simplified) that simply say... no engine above XX displacement. (so they can set the safety line to meet safety code)

Those can be some of the most entertaining events. Watch a .15 power plane eat steamers from all the .46 powerd planes (and have no trouble dancing around them) then the behemoth gets mad... and dives through the little guy... dragging home a "spare" RX on his fin.[>:]