Most slas require a dc check to be effective, however no dc for them is displayed, additionally nowhere are we given info on how the dc of the majority of slas is even calculated...
SO, we cant know if multiclassing them will still allow decent dcs. E.g. if the dc is calculated by character level + stat mod, or character level + x(whatever it may be) then a multiclass build could get decent dcs, however if the slas were based on caster level in the class that they were meant for (on some of them even that isn't apparent), then they would be near pointless for many multiclass builds.
Even worse, we don't even know what feats and increases will apply to any given sla, from massive testing I can tell you that energy burst dc is not increased by many things that youd think should increase it, ED evocation dc twist would increase it, but an evocation dc item would not? Same thing with caster levels, some stuff increased burst level, some didn't, no obvious rhyme or reason to it.
Which leads us to what I feel is the real problem, the reason dcs are not displayed on slas is that there are so many discrepencies and bugs that would be apparent if you did, is there any legitimate reason for not showing dcs on all spells that im not thinking of? I mean the dc must be getting calculated before each cast, so its no secret to the game.