Some of the challenges are identical for all industries when running fully-fledged PR campaigns and they are quite common. Namely, a misalignment in the project management triangle (scope, time, budget), misunderstanding of the audience needs or not using the adequate channels to reach the audience.
For tech PR in particular the challenges include lack of industry expertise for the PR expert – especially in what concerns the enterprise audience, lack of experience when planning the budget on the client side and, of course, lack of solid data when launching a new service or product on the market. But sometimes the challenges are less visible in the beginning and emerge only later on; for example, many corporate clients build their websites using technologies which are not particularly search engine friendly only to learn later that it costs them more to maintain or update it and especially to make it to perform in search results.