Manufacturers will tell you their connectors are only weatherproof when they have been assembled correctly (torqued up etc) and mated with a matching connector per instructions. For example, Staubli MC4 Evo-2 is IP68 when mated, but only IP2X unmated.

I have seen the after-effects of "MC4s" (from various manufacturers) installed in poor conditions without due care, or where damp is otherwise is allowed in, and from a large enough batch some will indeed fail. It is an issue the utility-scale solar industry is aware of at least. Whether domestic installers stop to think about it I couldn't say; as with everything in domestics I suspect it varies.

This does not necessarily mean that installs should never take place in poor weather, but that when it is, measures should be taken to keep the connectors dry until they're mated and checked. Indeed on larger constructions measures may need to be taken even in dry weather if it is likely to rain (or get dusty) twixt making up connectors and mating them. Fortunately the design of a made-off connector work in one's favour such that external damp surfaces can be safely wiped dry while the risk of water getting directly insde is more from being left in pools of water, on the ground or clipped in running streams, rather than an errant rain-drop under a light mist... but I would not (and do not) accept wet connectors being mated and am absolutely not above requiring cables to be reterminated where there is a risk of damp.

End caps are available if they are to be left exposed for a period, but less high-tech solutions are normally adequate for temporary situations.

Meanwhile if it's really hammering it down, is that the best time to be sat on a roof working with live 1000Vdc, from an EAWR perspective? Note again that until they're mated the connectors are only IP2X, and there may well be no means of isolation.

Hope that helps

Thanks @Jam, all the MC4 connectors have now been replaced, together with the attached PV panels and optimisers! It really helped to know where I stood, from your excellent advice.

The solar company eventually agreed to bag every new MC4 half-connector individually, in case of rain (not onerous), until just before mating the connectors. However each worker (roofer, roofing manager, & electrician) that arrived one at a time to do the replacement, still argued against it!

The solar installers have proven to be a really honourable company, IMHO. This year (2023) they have the council contracts for Solar Together Suffolk, Norfolk, and Bedfordshire.

Thanks @Jam, all the MC4 connectors have now been replaced, together with the attached PV panels and optimisers! It really helped to know where I stood, from your excellent advice.

The solar company eventually agreed to bag every new MC4 half-connector individually, in case of rain (not onerous), until just before mating the connectors. However each worker (roofer, roofing manager, & electrician) that arrived one at a time to do the replacement, still argued against it!

The solar installers have proven to be a really honourable company, IMHO. This year (2023) they have the council contracts for Solar Together Suffolk, Norfolk, and Bedfordshire.