It is a significant disadvantage though difficult to quantify exactly how much. There are two aspects of the question leaf springs and solid axle.
The solid axle is a disadvantage vs IFS primarily in terms of (i) significantly higher sprung vs unsprung ratio (ii) No negative camber adjustment possible (iii) Limitations on increasing wheel travel. The Evos have independent rear suspensions as well...
Leaf springs at the back for longer vehicles are quite alright and work well as Nadir's Tacoma, the Vigos, Ford Raptors, desert racing Cherokee XJs all run leafs at the back albeit at the higher end these are customized to increase travel (even upto 15-24 inches!). With multi-stage damping shocks (triple bypass) readily used now, it is apparently possible to overcome some of the inherent shortcomings of leaf springs. Leafs at the front are just a pain....
All said and done a solid axle leaf sprung vehicle will have some positives over IFSs: (i) can generally withstand much more abuse before suspension breakdown (ii) easier and cheaper to calibrate spring rates according to vehicle weight (iii) relatively less load on the shock absorbers limiting shock fade in a long rally (iv) lower maintenance requirements in the long run.