What "downside" is there? Less blight? Some jobs? Tax revenue? A general improvement in that part of town?

A year or two of construction traffic and (typically) a lot of noise. Data centers are loud (lots of external fans and exhaust), and they run 24/7. You're basically replacing a commercial use surrounded by parking with a light industrial use coming up to a few hundred feet of the property line, so you're materially altering the noise impact especially at night (when commercial uses are not usually operating). That's particularly bad here, because while 90 acres seems big, it's actually small for a data center - so this plan has relatively modest setbacks of only a few hundred feet from the property boundary.

You also get air quality issues, because these facilities typically require on-site full back-up generators. They're usually diesel, and they're usually scaled large enough to cover the entire load of the data center. For the 800+ MW data center proposed here, that's basically a small power plant right in a residential neighborhood - one that gets turned on only intermittently, but which isn't going to have the same pollution control technology (since it's a back-up) as a "normal" power plant that size.

Aesthetically they're not particularly attractive, either. Yes, better than a vacant lot - but a vacant lot might one day get redeveloped, while the industrial use serves as a deterrent against other non-industrial uses coming anywhere nearby.

Then you have all of the regional impacts - using up electrical grid capacity, some impact on water availability. People are genuinely concerned about those.

I don't have a problem with data centers generally, but I think that from a land use perspective you want to put them on large remote parcels far from residences. Sticking them on infill lots in urban areas is probably not a great idea, and usually light industrial is not considered a compatible use for many of the places where commercial zoning currently exists. These objections aren't crazy.