Here is an article about the probably unavoidable downsides of fusion [0], from the Bulletin of Atomic Scientists. To summarize their points:

- neutron bombardment, the unavoidable consequence of the only realistic fusion reaction, deuterium + tritium, turns every known material brittle and radioactive; and can be trivially used for uranium enrichment

- tritium is almost impossible to contain, and requires fission plants to create - the fusion plants could theoretically create a surplus, but they would have to recapture essentially 100% of a very hard to capture gas (today they normally leak about 10% of the injected tritium)

- fusion plants need to be massive to be even close to break-even - the energy drain of the facility consumes a sizeable portion of the generated energy; most of this power drain is still required while the facility is not operating the reactor, increasing projected costs of running a facility

- ICF tech is very close to fusion weapon tech, so there is a massive risk for proliferation from there as well; MCF tech is not, thankfully

[0] https://thebulletin.org/2017/04/fusion-reactors-not-what-the...