Right from the official page:

Celsius Network is a leading Centralized Finance (CeFi) platform for crypto assets offering competitive interest reward rates for deposits and low interest rates on collateralized loans. Over 40 assets are supported and the community has grown to over 1 million users in just 3 years.

This offering is a bit of a strange one, it's a centralized finance platform that takes decentralized assets as collateral. I see this as being mostly backwards but whatever.

Calling traditional centralized finance CeFI is exceedingly annoying because this is clearly a linguistic trick to make the term sound more like DeFI which is frankly the complete opposite of this.

I think the biggest problem with Celsius is that the insanely high interest rates being offered just won't survive a downturn. Like many things in crypto the issue is that extremely high amounts of leverage and an constant influx of money into the ecosystem is making many things that are obviously unsustainable look profitable in the short term. The extreme growth in monetary base is what allows the high interest rates to be offered. But what happens when the inevitable deleveraging occurs?

As the saying goes "you can't taper a ponzi scheme".

EDIT: Update from 2022 May 15

Looks like Celsius is now facing massive pressures due to the unfolding crypto market collapse. LUNA was a top 10 by market cap earlier in the year and is now completely worthless. TerraUSD which is the associated "stable"coin has pretty much completely been wiped out as well since the peg to the USD failed. These feel like the some of the early dominoes falling as part of a huge crypto market wipeout. LUNA has now stopped trading entirely and the blockchain it runs on has been completely halted. In a move reminiscent of the Nickle market trading on the LME¹, some exchanges have even retrospectively cancelled trades on LUNA, further screwing the public.

In April 2022 there was talk that Celsius had regulatory risks, effectively concerns that the network was providing unregulated securities could lead to regulatory action to shut the network down. Now in mid May 2022 the situation looks to be further deteriorating with the general crash in crypto. Many people who jumped in on the crypto during the hype and mania cycle are now feeling fearful and wanting to withdraw all their money. Crypto schemes that don't have sufficient collateral to back the deposits will be in big trouble dealing with these outflows. This in many regards has much of the dynamics of a bank run.

We have had the CEO of Celsius claim that there's no major losses despite the high volatility in the sector. It seems that if enough people withdraw funds all the companies that had insufficient capital reserves will go under. In the crypto space this is likely to be the end of a lot of organizations since shady accounting and crappy collateral tends to be widespread.