Allergic reactions are possible for any medication. Nobody would want you to take a medication you are allergic to. However, allergic reactions to biologics are comparable to allergic reactions to corticosteroids.

"The cumulative incidence of hypersensitivity reactions over 6 months for all biologics was low (< 1%)."
https://pmc.ncbi.nlm.nih.gov/articles/PMC5420339/
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This is comparable to hypersensitivity reactions to corticosteroids. I'm more concerned about long term exposure to corticosteroids. The adverse effects of long term use of corticosteroids are well documented. My 12 years of continuous daily use of Prednisone weren't exactly uneventful.

"Hypersensitivity reactions to corticosteroids (CS) are rare in the general population, but they are not uncommon in high-risk groups such as patients who receive repeated doses of CS."
https://pubmed.ncbi.nlm.nih.gov/23567983/
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I don't think there is enough data regarding long term use of Actemra but I haven't had any allergic reaction or serious adverse effect to it in over 5 years. People react differently to medications so what is good for one person might be bad for someone else. Fortunately. it is very easy for me to discontinue Actemra if something goes wrong. The same can't be said for when I was taking Prednisone.
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I'm actually more worried that my body will form antibodies against Actemra and make it less effective or it might stop working altogether.
https://www.verywellhealth.com/antibodies-to-biologics-4846279