Stalinism killed many people for several reasons that probably aren't relevant to "modern socialists." But some of the main reasons are still relevant. "Modern socialists" are still advocating the nationalization of industries, having vast amounts of economic activity being directed by the government, etc. (Ironically for Corbyn, some of his ideas will only be possible if the U.K. brexits, because of the E.U.'s deregulatory posture.) Those ideas are bad for the same reason Mao's collectivized farms killed tens of millions of people. Central planning is a bad way of running the economy. Businesspeople are better at running industries than government bureaucrats or political scientists.

2019 manifestos are not yet available. So I guess you are thinking of the Labour policies in the 2017 election. Which policy was only possible after Brexit? The IFS (Institute for Fiscal Studies who fact check all public finance here) were critical of both 2017 manifestos, but were far more directly critical of the Tory one as being damaging to the UK economy.

It is a very long time since the Tory party could be considered a safe pair of hands economically.

But Stalin. But Lenin. But Mao. But Pol Pot. But Chavez. But Maduro.

Too many power-hungry zealots; too little competent governance. Too much destruction of the old order; too little creation of a decent new one.

[Edit: But Xi. No, he hasn't destroyed the economy. I look at the Uighurs, though, and put him in the "murderous power-hungry zealot" category.]