ETF ISXF...reasons to buy/not buy?

It depends what you mean by lower risk.

If equities crash 40% on global growth concerns you would not expect it to crash by the same amount. However, the nice smooth growth chart of 50% in the last 5 years is not going to be sustainable with investment grade bonds only paying a few percent per year and already priced relatively close to all time highs; there isn't another 50% of headroom available for the next 5 years.

Instead, you should consider what price those bonds would be valued at if GBP interest rates and other global interest rates increase by a few percent - how much lower would the same investment grade corporate bonds be priced than they are today, if base rates move up from 0.5 to 3.5%, and inflation increases, and nobody needs to pay the same crazy prices for equivalently risked fixed income streams?

The answer might be that the fund is destined to lose capital over the coming years, but you might be happy with that if you don't think it will happen (or not that quickly) and are happy to take the extra yield it offers over a cash ISA.

It is at least physically replicating the index rather than using mostly derivatives, hence perhaps lower counterparty risk than some ETF options although could still have liquidity problems in a bond crash.