The other extreme would be INF+PWL as it's so cheap and takes so little space that you can bring millions of them easily, though of course casualties would be horrendous.
Given "balanced tech", i.e. equal racial attack and armour compared to an opponent, INF+PWL by themselves are actually quite bad for anything other than boarding combat (but usually very good at this one thing). Compared to INF+PW, they will have about 42% of the kill rate (25% on a per-unit basis, but +67% more units for the tonnage) and 60% of the loss rate (3 tons lost per death vs 5 tons for INF+PW). This is not a good ratio and they will quickly fall behind against INF+PW, INF+CAP, to say nothing of the extreme armored units you can build. Note that since ground force costs are on a per-ton basis (multiplied by armor and capabilities where applicable), INF+PWL are
not cheaper than INF+PW per ton, only per unit which is not a very meaningful measure.
INF+PWL is most effective when you want to delay the enemy as long as possible, either as a cheap meatshield for your real killers (UHV+4xCAP) or to screen your artillery. It is pretty debatable whether either use case is more effective than plain CAP spam, probably not against the NPRs but against a player race or possibly some spoilers there may be benefits to such an alternative approach.
It is worth pointing out, by the way, that in ground combat the effect of formation composition is relatively minor as long as you are sensible about it - against NPRs this means lots of infantry and/or multi-shot weapons (CAP, LB, LAC, and so on) as your main components. Usually the relative tonnages involved in combat (you need 3x to 5x advantage to invade most planets, at a minimum, and more is better to reduce overall losses) and technology which scales to the fourth power in theory and somewhat less in practice but still following a rough, superlinear power law behavior. This is one way INF+PWL can be actually more effective than usually expected, if you put a lot of RP into plasma carronades to boost their attack beyond 2x the enemy racial armour tech you can achieve actually better effective kill rates than INF+PW/CAP. Personally I consider abusing plasma tech against NPRs to be rather gamey but each to their own I suppose.