TAS actually had an unusually high budget for a 1973 Saturday morning show, though I think a lot of that went to paying the actors. Its first season, conversely, was made under an insanely tight deadline for animation -- 6 months to do 16 episodes -- so the work was more rushed than was ideal, and that worked against its quality somewhat. The 6 episodes produced for the second season are a bit more polished, I feel.
Writing-wise, it was terrific for Saturday morning, and I'd say a lot of its scripts were competitive in quality with a lot of TOS's scripts. Acting-wise, the actors with voice-work experience (Doohan, Takei, Nichols, Barrett) did well enough, but the three leads were unfamiliar with the discipline, and because of the scattershot way recordings were done (literally mailing in their parts from different studios around the country), they didn't have a single director to guide their performances, so that undermined their acting.
Flimation is noted for their cheapness
I prefer to call it economy. They found ways to do more with less, and I think the quality of their work was superior to that of their main competitor, Hanna-Barbera. Much like Roger Corman, their productions may have been bare-bones, but they were a training ground for people who'd go on to do greater things. A number of the people who revolutionized TV animation in the '90s got their start at Filmation in the late '70s and early '80s, including Bruce Timm.
TAS had some well written episodes, but other episodes had juvenile writing
The writing was never intentionally aimed at children, except in terms of reducing the sex and violence. It does get silly at times, but so does TOS.
and the whole series as plagued by a tiny voice cast
Small repertory companies were routine for Filmation, and for other animated works as well. Most of Hanna-Barbera's voices in their early cartoons were just Daws Butler and Don Messick. Jay Ward's shows had most of their voices done by Bill Scott, Paul Frees, June Foray, and William Conrad. Mel Blanc alone did the vast majority of Warner Bros.' theatrical cartoon voices, along with Foray and a few others. A number of old radio series got by with fairly small ensembles doing a lot of different voices too, or at least a few lead actors and some versatile character actors doing most of the supporting cast. So I wouldn't call it "plagued" by any means. TAS actually had a larger cast than typical for a Filmation show.