It varies heavily by institution and country, but CS is moving increasingly towards caring about citation metrics above anything else (with "selectivity", i.e. a high bar for peer review and low acceptance rate, being the main other factor). Unlike in most other fields, conference papers therefore hold weight, not only journal articles. This does sometimes cause trouble at higher levels of large institutions, where a CS dept strongly recommends a candidate for tenure, but when the case makes it up to the dean level, the dean, who is a physicist or biomed person, wonders how they could possibly recommend tenure for someone who has "just" a bunch of conference papers and few journal articles. But that is becoming rarer at places with top CS departments.
Anyway, as a result, I don't see a reason why an alternative-format journal would necessarily fare any worse than conferences have in terms of becoming accepted, if the reviewing standards are high and if it attracts citations.
For the hiring side (more than the tenure side), to some extent, oddly enough, the first-order decision here is in Google's hands. A lot of CS hiring committees nowadays unofficially do a first cut sifting of resumes by typing candidates into Google Scholar and looking at their Google-computed h-index, so what "counts" is basically up to Google.
