Weight: You're adding about 500 lbs to the car to do this with ~20 miles of ev range. Suspension may need to be altered to support this.
Practicality: the batteries have to go somewhere and they can't go under the hood since the engine is still there, so they may go in the bottom of the trunk. From the examples I've seen there's usually less cargo capacity, and the spare tire is removed. Some vehicles that are already hybrids (e.g. Toyota sienna 2022-) have an already tight layout with exhaust and all that it becomes a design challenge to fit a larger battery.
Thermals: a li-ion battery for an EREV likely needs thermal management so it doesn't lose range prematurely, so you either need a dedicated cooling line setup that works with the ac compressor (bmw i3) or an aircooled setup where the battery is in the trunk and relies on cabin ac/heat (various toyota hybrid models do this).