If you are starting a freelance product description writing business, do you generalize or specialize? In other words, do you offer writing in an area like "product descriptions for tech companies" or does that limit business too much?
This is a form of copywriting (ad writing) and I'd say broadly specialize, but not by label. Mention broad types of products you feel good at writing about. Tech products, household appliances, kitchen appliances, automotive add-ons are examples.
Don't exclude anything or "go negative" in your ad, if somebody wants to contact you about art supplies, or fashion products, maybe you can do it.No need to proclaim all the things you refuse to do, that just makes you seem difficult to work with for legitimate customers that might have called you.
Specificity helps sell, but more important in copywriting is to write about how your service alleviates some (usually metaphorical) pain they have, or represents a new opportunity for gains they want. The value of product descriptions is saved time on their part, a focus on balancing brevity with clarity and accuracy, so you don't lose sales because the prospect's questions were not answered before they became bored or confused. (lost sales is the "pain"). Your service is fitting the best description possible, prioritizing the critical selling features, in the space allowed, by the catalog, the letter, the postcard, the Internet ad, the email, whatever it is. To maximize inquiries, sales, traffic, or whatever the customer needs.
I would make a description of the kinds of products you service (one per broad category) secondary to this, just to reassure your prospects you understand the different approaches to different categories.