The most obvious is that it uses resources that could be used elsewhere. The amount of resources spent in every election is massive. Groups / orgs that do electoralism often spend most of there resources this way, because they have to compete with way bigger and better founded parties.

Also doing electoralism often is something of an image and credibility thing. To have chances there groups / orgs often try to have a civil / good / appealing image and they often care about that image enough to not do radical / bad optics actions.

Also when you do electoralism you agree to certain conditions of the state to participate. In most states you have to have certain legal structures and processes in place which usually are pretty hierarchical and are really hard / impossible to work around in horizontal and antiauthoritarian ways. This way it makes it (nearly) impossible to implement anarchist concepts such as “unity of goal and practice” (sorry I dont know a better translation, maybe prefiguration fits). Agreeing to be a legal entity that the state recognizes and allows to participate in elections also reduces the field of activities you can do without impacting your ability to do electoralism.